So, Boimler graduated the Academy in '79 (same year as Nemesis), so he's in his early 20s in Lower Decks, assuming he's not a child prodigy like Wesley.
Rutherford and Tendi seem similar, Mariner is older by nearly a decade (she's been serving since at least 2372, or season 4 of DS9), so she's late 20s, maybe 30 by Lower Decks Season One.
This also means that Boimler would be nearly 39 in Picard season one.
For other points of comparison: Boimler is actually younger than Jake Sisko. Mariner is about the same age as Harry Kim or is Wesley's Age (and she was a child prodigy like Wesley given she was younger than Sito). Incidentally, this also means that Mariner is slightly older than Raffi.
That's assuming we're not going with the actor's ages, which screws everything up and makes Boims a late arrival at Starfleet Academy at 27 and Mariner around 40 in Season 4 or four years older than Wesley, which would have her be about 20 in Starfleet Academy flashback (which is actually quite reasonable)
We know that Picard could be a Lieutenant Junior Grade in his sixties too so this isn't necessarily a mistake.
Rutherford and Tendi seem similar, Mariner is older by nearly a decade (she's been serving since at least 2372, or season 4 of DS9), so she's late 20s, maybe 30 by Lower Decks Season One.
This also means that Boimler would be nearly 39 in Picard season one.
For other points of comparison: Boimler is actually younger than Jake Sisko. Mariner is about the same age as Harry Kim or is Wesley's Age (and she was a child prodigy like Wesley given she was younger than Sito). Incidentally, this also means that Mariner is slightly older than Raffi.
That's assuming we're not going with the actor's ages, which screws everything up and makes Boims a late arrival at Starfleet Academy at 27 and Mariner around 40 in Season 4 or four years older than Wesley, which would have her be about 20 in Starfleet Academy flashback (which is actually quite reasonable)
We know that Picard could be a Lieutenant Junior Grade in his sixties too so this isn't necessarily a mistake.