I do not think there is an optimal age. Legally speaking above 21 for sure.
I guess it all depends; if you are 22 years old and there’s no hollowness or sagginess, I don’t see why you need a filler or anything at all unless you have some acne scars, for that age range, usually acne scar is a problem.
But, let us say you are 25 and have a hollow, sunken temple then a temple filler might be suitable for you.
