過去スレで>>267と全く同じ質問を見かけましたが同じ人ですかね？
とりあえずRaces of the dragon p10より

AGE
A dragonborn character is reborn, losing any bonuses or
penalties she received because of her age. She emerges from
her transformation as an adult. Dragonborn are hearty and
capable of living long lives. Due to their perilous fi ght
against the spawn of Tiamat, however, most die young.

要するに転生前の年齢ボーナスもペナルティも取り消されます
これは過去スレでも同じ回答です