I only tentatively agree about Mei. Having her be frozen is a little goofy and just comes off as "we wanted her to be cute but still somehow have ties to Overwatch while it was active/Overwatch in the past".
Which... is why I think there may be all these contrivances in the first place. I think they had specific designs in mind for their characters, but wanted to tie them back to Overwatch while it was still active.
The other examples aside from Mei, though; I think they mostly have solid justifications for their ages if we're not being completely cynical.
Tracer has the time travel thing as absolutely core to her character, and she plays as a very fast and quick character which I think is fair for her being youthful.
Pharah has issues with her mother that we haven't seen the full extent of, and it's possible her mother is coming back as her own character, so I don't see her as a just a "replacement"; there's more hiding in there.
Widowmaker is for better or for worse going for the Femme Fatale trope, which makes her appearance a bit obvious but also expected (that could be it's own separate discussion, but it's not an uncommon concept to portray at this point).
Mercy is one of the most interesting characters in the game and there are many mysteries she's got her hands in, including her distinct lack of aging.
D.Va's a pro-gamer-turned-mech-pilot, and the actual pro gaming industry is notorious for it's lack of older players and people seem to age out of it very quickly; changing her age would mean changing her core inspiration.
Symmetra could have been designed as older I suppose, but I think her current age plays into the idealism she harbours for her company and the future, despite some "ends justify the means" projects they seem to be getting up to. As for
Zarya there hasn't been much info on her back story IIRC, or at least she seems fairly isolated from the rest of the cast, so that could very well be an arbitrary choice as well.
And this is where these kinds of discussions go a little sour for me. Very similar to the "Female Link" thread, we're stuck discussing hypotheticals about the design process which we don't really have much business directing or having a say in from the outside looking in IMO. Yes, they could have had a female Link, but decided not to (reasons publicly given were poor, but it still stands). Yes, they could have designed some of these characters as older, but they decided not to. Saying "they have a diverse cast of women but no one is allowed to look older than 30" feels extremely nit picky to me. Would you have preferred some of the existing cast to be older and compromise their design vision for that character, or have one of these women cut for someone of a completely different design? Trying to think of who was "arbitrarily" "made younger" and who wasn't because they're "not allowed to look old" seems like a fool's errand to me, because we weren't privy to the design process behind the cast.
It's worth nothing that no robots or true cyborgs in the game can be female.
I don't think you can say that that with such certainty. Obviously there are none in the game as playable characters yet, but in the universe there very well might be, there is still a lot we don't know. Or are you just referring to the robots as being genderless in general? Zenyatta seems to be quite clearly portraying a male personality. The Athena AI has a female countenance and is another character rumored to be coming down the line.
Overall I think this is something that might be worth discussing, but I don't think it should take anything away from Blizzard's pretty fantastic and diverse cast that they've already made, which is what the OP seems to be suggesting in the second last line. I think bringing this up as a discussion could make some room for
future characters to show this side of the team, but picking apart the cast as it stands doesn't do anyone any favours.