These are rough estimates but, by Avengers 1's release date in 2012:
Scarlet Johansen (Black Widow) was 27,
Chris Hemsworth (Thor) was 28,
Chris Evans (Capt. America) was 31,
Downey Jr (Iron Man) was 47,
Mark Ruffalo (Hulk) was 44
Jeremy Renner (Hawkeye) 41.
Avengers 2 was released in 2015:
Elizabeth Olsen (Scarlet Witch) was 26
Paul Bettany (Vision) was 44
Avengers 3:
Chris Pratt (Starlord) was 38
Zoe Saldana (Gamora) 39
Dave Bautista (Drax) 49
Tom Holland (Spiderman) 22
B. Cumberbatch (Dr Strange) 41
Chadwick Boseman (Black Panther) 40
I see a trend I like here.
By Avengers 4 we'll get:
Brie Larson (Captain Marvel) 28
If we get X-Men in Avengers 5, I am hoping to God they are all in their late 20s and up because we need a mature X-Men team movie!!! Fox never gave us one with a mature Cyclops, Beast, Rogue, Storm, Gambit, Wolverine, Psylocke, Angel, Colossus, Jean, etc.
New Mutants has a great young cast already and they have several important characters yet to be cast, including: Magma, Karma, Cypher.
Also, the X-Men usually have ONE rookie to put on their team or to take as a sidekick in Wolverine's case. We've seen Kitty Pryde, Jubilee, Rogue and X-23 (in the Fox universe), Armor in Whedon's run (I never liked this character by the way), we've seen Colossus' love for his little sister Magik as well. But we don't need all of them to be in their early 20s.