>>56337459A Mega Stone and Mega Ring/Band/Accessory is needed to Mega Evolve, not all Gardevoir will ever come in contact with one/both of these.
A better question: If a Pokemon chooses not to evolve or they, somehow hold onto/merge with an Everstone and thus cannot evolve- would it be appropriate to be romantically involved with them? You could meet a Ralts or Kirliia who's in their 70-90s age-wise and has lived a long life but is happy as or is stuck in the body of a Ralts or Kirlia.
Also, what do you think an older Gardevoir looks like?