How to translate またひとつ成長する?

[raintree_leaf]

I always find counters like ひとつ hard to translate since it is heavily context based.

[LavaBug]

To grow another bit?

Hard to tell without having the context...

[Meia_Gisborn]

ひとつ　一つ　
One, a, an, one year...　テキストが違えば、色々な意味があるね。

[Yiuel]

I would go with "To get one year older" without context.