This is far from true. Improving as a shooter as a player ages is the norm, not the exception. Look at Magic's three point percentage when he got close to 30. Or you can check out PP, Nash, Dirk, even Larry with all the injuries improved his outside shooting at that age.
Not sure what you mean - Pierce shot a career high .412% from 3 his rookie year. He's been a bit inconsistent throughout his whole career, but it's always been there. When he was 24, he shot 40%, his fourth year in the league.
Dirk shot 38% his 2nd and 4th year in the league and that has been his average for his career.
Larry shot 41% from 3 his rookie year, fourth highest of his career. Yeah, he shot a bit more consistently in the second half of his career, but the ability was there early on.
Point is - it's very rare that someone comes into the league with an already terrible shooting ability and really improves on it.
I think Magic/Kidd are exceptions to the rule. Would love to see Rondo be another one though.