Beware, older languages below! The languages below were invented during Tolkien's earlier period and should be used with caution. Remember to never, ever mix words from different languages!

Qenya 

inárea

as old as

The correlative ᴹQ. inárea “as old as” appeared in Demonstrative, Relative, and Correlative Stems (DRC) from 1948 (PE23/111), a combination of the relative pronoun ᴹQ. i and an adjectival form of ᴹQ. are “day”.

Neo-Quenya: For purposes of Neo-Quenya I would instead define a neologism ᴺQ. yanaurë “on the day when” using the relative pronoun ya and aurë; compare Q. yallumë “in times when”. I would then use its adjectival form ᴺQ. yanaurëa for “as old as”, more literally meaning “of the same days as”. For example: nás yanaurëa ní “he/she is as old as me, (lit.) he/she is of the same days as me”.

inárea ní

as old as I am