You aren't supporting the original assertion, which was:
"evidence suggests little guards don't age well"
All you are doing is repeating what we already know and is not being debated: That there are few short players that are good enough to play in the NBA at all, let alone be 20 point scorers.
You've also pulled 20 ppg out of your hat as the magic threshold of goodness.
Nothing you have posted shows that a player of a certain level of production is going to decline faster in his production if he is under 6 feet than if he were over 6 feet.
You've gotta be joking.
Short players NEVER maintain 20ppg production well into their 30's.
This is where you just don't seem to understand the question. You don't seem to understand how such a statement like that is ridiculous.
There have been only 5 players ever who were under 6' who ever averaged 20 ppg in even one season, and one of them is Isaiah, who hasn't reached the ripe age of 30 yet.
Of the other four, Dana Barros only managed 20 ppg once in his 14-year career (age 27) and otherwise never averaged more than 13.3 ppg so expecting him to be a 20 ppg kind of player later on would be clearly pretty silly, whether he was over 30 or not. Same with Michael Adams, who had a fantastic age 28 season as a scorer (26.5 ppg) but never cracked 20 in any season prior or after in his 10 year career. And again, the same with Damon Stoudamire, who came out of the gate as a good scorer, scoring 19.0 & 20.2 ppg in each of his first two seasons, but after that, never came close to being a "20 ppg scorer" in the rest of his 12 year career.
None of those players had any '20 ppg' level of production to "maintain" into their 30s.
Of the 10 total seasons of 20 ppg by short players, seven of them belong to just two players: Calvin Murphy (5) and Isaiah Thomas (2).
So our data set of short players who are "20 ppg scorers" with which to compare Isaiah to (to project his future with) amounts to ... one guy: Calvin Murphy.
So, from our 'data set of one', our one and only "20 points per game" scoring short guy DID manage to maintain 20 ppg rates for at least two seasons into his thirties.
Almost never even 15ppg. 10 ppg is something of a rarity. There's nothing magical about 20, pick any number you want.
Sure, again, though, we still end up with very tiny data sets.
If we use 15 ppg, we end up with a total of just 11 players, of which only 6 ever repeated that feat more than 3 seasons, of which two (Isaiah, Ty Lawson) are still playing and under age 30 so that leaves us a comparison pool of just 4 players in the data set (Stoudamire, Calvin Murphy, Terrrel Brandon & Michael Adams). Of those 4, three posted at least one season of 15 ppg over the age of 30.
If we drop the threshold to just 10 ppg, we get 12 players who have posted at least 3 seasons of that high of per-game scoring, of which 10 are not still playing and under age 30. Of those 10 players, 7 played at least one season over age 30 where they scored 10 ppg.
Interestingly, several of the short players who only managed one or two such 10 ppg scoring seasons, did one _after_ reaching age 30+: Bogues, Al Cervi, Buddy Jeannette, Chick Reiser & Mel Riebe.
Taller players are commonly able to maintain such production. About half of 20 point scorers over 6 ft are able to maintain that production after 30. Nearly ALL of them continue to score at least 15. Yet there has never been a player under 6 foot to produce at a 20pt level and only 4 seasons total where they even scored 15.
Umm... There have been 227 players of 6 feet or taller who have posted at least 2 seasons of 20 ppg. There have been just TWO short players who have done that. One of them has not reached the age of 30 yet. Yet you are trying to compare the rates of fall-off in 20 ppg production?
Do you understand at all how statistics work?
Don't twist this into some obtuse rate-of-decline double-talk. I just said players decline physically at the same rate. What I'm talking about is "can short players produce after 30"?
How is it that I am 'twisting' anything when I am staying consistently on the original assertion and it is you who are trying to change the goal posts?
That's the assertion I'm supporting. No, they can't. Not at elite levels. It has never happened. The absence of evidence is its own form of evidence. The reason you've never seen a unicorn is because they don't exist. You don't need Pelton's unicorn % calculator.
Okay, it's pretty clear that, indeed, you (a) love to move goal posts and (b) do not understand how simple statistics work.
You can't say 100% of a population will never do X if you have almost no data in the sample.
It especially sounds silly when the one data point we have (Murphy)
did do X. I.E., 100% of the 20 ppg scorers who were under 6' managed to post 20 ppg seasons over the age of 30.

Can tall players continue to produce at all star levels? Yes, sometimes. In fact most of them DO produce at appreciable levels well into their 30's. There's a heavy decline after age 34, by which time most small players are no longer in the league.
Again, all you've shown here is that the selection pool of small players is ... ridiculously small. Most small players never even got _into_ the league, let alone to be able to last past age 34. This says absolutely nothing about whether a small player of production X is going to age faster than a tall player of production X. Because there have been very, very few small players, there have been basically only one small player who has been a repeat '20 ppg' scorer (prior to Isaiah). So the fact that there is only one such player over age 30 compared to hundreds of taller players doesn't say _anything_ about rates of decline or the ability of another short 20 ppg player to still potentially produce after age 30.
One of the problems with your approach is that you are focused around ppg, as if points were the only value a player brings. This arbitrarily reduces the pool of players because it removes players who bring value to the game in other ways. If we expand to Win Shares, that will bring in the value of assists, rebounds, steals, blocks, etc., into the production and allow us to increase the player pool size for both tall and short players. Logically, because of rebounds, it should favor taller players, though maybe assists & steals can even some of that out for the little guys.
It's important not to get bogged down in a debate over the merits of Win Shares. They are used here simply as a course overall measure of production. We could do the same exercise with Wages of Win or other such roll up and I'm pretty certain the results would be similar.
Most folks consider a 5 WS season to be a pretty good season - not all-star level, but very productive and valuable. Avery Bradley produced 4.8 WS this season, to give you a comparison point. Only Amir, Jae & Isaiah produced more than 5 WS this season for the Celtics, so that level represents a player who is, "one of the better players on a playoff team". A WS total of 7+ starts to be on the fringe of all-star level, and 9+ is solid all-star level.
To make sure that all players in these next samples have reached well past the age of 30, I'm restricting the filter to players who debuted in 2000 or earlier. That leaves off a few players from the next couple of classes that came into the league 'old', but I don't think that matters.
There have been 450 tall players, 6' or higher, in the set who have posted at least 3 seasons of at least 5.0 WS. Of these, 198 (44.0%) posted two such seasons after age 30 and 318 (70.7%) posted one of them after age 30.
There have been 11 short players, 5' 11" or shorter, in the set who have posted at least 3 seasons of at least 5.0 WS. Of these, 5 (45.5%) posted two such seasons after age 30 and 8 (72.7%) posted one of them after age 30.
You mentioned age 34 -- yes, there is a huge drop-off after this age. For all players.
Of that set of 450 tall players up above, only 20.6% managed to post at least one 5.0 WS season after age 34.
And of the set of 11 short players, only 18.2% managed to post at least one 5.0 WS season after age 34.
Maybe Isaiah can be the next little guy data point to bump that up to 25.0%?
All these suggest that the age vs production rates are at least similar between taller and shorter players.
This data also suggests that there is a drastic selection criteria to even get into and become a regular, productive NBA player that is heavily and understandably slanted against shorter players. But that once such a player HAS the criteria (skill, athleticism, etc.) to be able to stay and produce in the NBA, his age vs production projection is probably no different than the average tall player of similar production.