Learn Portuguese (Portugal): Já não tenho idade para estas festas.

While you could literally translate "I am too old" as sou demasiado velho, Portuguese speakers rarely phrase it that way. The natural idiom is ter idade para (to have the right age for). Saying you "no longer have the age for" something is the standard, colloquial way to express that you have outgrown it or are too old for it.

Why use já não instead of just não here?

Já não specifically means "no longer" or "not anymore". If you just said Não tenho idade para estas festas, it could technically mean you are too young! Adding já makes it clear that you used to be the right age for these parties, but that time has passed.

Can I use ter idade para in the affirmative to mean "old enough"?

Absolutely. Just as já não tenho idade para means you are too old, the affirmative version means someone has reached the appropriate age or maturity. For example, Ele já tem idade para conduzir means "He is already old enough to drive".

Why is there no article before idade? Shouldn't it be a idade?