Mass Effect 1 - Shepard is 29
Mass Effect 2 - Shepard is '31'
Mass Effect 3 - Shepard is '32'
From Hero to Legend.
Companions ranged from barely matured, to rather old and experienced, but I'd say that more of them leaned at least towards the latter. While there were the younger and fresher characters, the story IMO seemed to focus more on a collection of individuals drawn to Shepard and contributing their personal skills and experiences.
Mass Effect: Andromeda is instead a soldier/ordinary? to Hero. 'Greener' team.
I can imagine:
-Protagonist starts in early or mid 20s, late 20s at the max. Enough to generally take that place what Shepard was at when he had his 'Service History' we input.
-MEA will basically be the elaborated history of this new character, again the journey to becoming a hero, so it lets Bioware establish them as arguably 'better' than Shepard, earlier on in their life. Of course there may be narrative counterbalances to this, like MEA's character not being as confident or secure in their skills and environment, but they'll still get details of their life that may be 'superior' to Shepard already before age ~30.
-A greener team does not mean incapable. It means that instead of (RELATIVELY) bringing baggage with them to the team, they may instead collect their baggage while in the team. There may be a greater theme of organizational unity. And while they may end up leaning inexperienced (think Cora? may be more Ashley ME1 and less Miranda, more Grunt and less Wrex), they'll still hold their own and for all we know end up becoming masters by the end of the game, and there is nothing necessarily stopping characters with experience from joining (even if less of them do compared to ME1-3).
I presume MEA's deal will have the most 'established' crew yet - even more put together than how Cerberus tried to establish the Normandy SR-2 - but there will be a great emphasis on how having even the best of the best planning may go bad and the people with the best aptitudes may be challenged in ways they were never programmed for. Sure their superior (for the Milky Way standards?) foundation may be great enough to give them a shot in the Helius Cluster, but the whole point is that we get to see strange new worlds with challenges that may never have been prepared for. Thus, 'green'.
I dunno if Bioware has any considerations on whether there will be more games with Ryder after this, but at the very least, I expect their relative status to start around 'Service History' (with us choosing Pre Service History in options, which might be where the family stuff comes in) and ending either at the end of the 'Service History' (localized incidents) or Ryder's own 'Mass Effect 1' (galactic scale adventure in some sense) or a mix of both. By that I mean, we may be similar in ways to the Inquisitor who may or may not be playable/recruitable/meetable after their feature game (DAI), but nonetheless goes through formative experiences in at least the one game (DAI/MEA).
I have no problem with playing a protagonist who may be in their 20s. Anything 20s-40s I can take, or even theoretically 50s+ in a setting that has at least youth extension technologies or any other useful story devices. Just as long as they are 'generically adult' enough in age portrayal (not 'whiny kid' or 'wheezing geezer').
There are concerns about the possible relative youth compared to Shepard, if only because I really don't care to do his story over again (and details are sometimes getting too close to that for comfort), but I know one thing that'd differentiate them to Shepard - more use of transhuman technologies. Let me use more overt implants and directed evolution/engineering on myself and I'd love that and certainly feel like a new character. However, I've seen very little direct evidence of this happening so far. Just stuff like possibly stronger biotics, jump packs and the like. Boooo.