So in House of M Magneto overthrew the government in 1979, and House of M takes place 30 years later (2009—this is in that newspaper thing included at the end of the trade paperback.) So then the twins would be in their 30’s. Judging from the fact that there’s a huge controversy over the fact that…
This is pretty interesting, but House of M takes place in an alternate reality designed by the Scarlet Witch, so I wouldn’t take it as something completely indicative of 616 canon. The twins were also born shortly after the end of WW2, as per Magneto’s backstory and his split from Magda. They were placed into stasis by the High Evolutionary, and then joined the Avengers in the mid-60s with Captain America’s unfreezing. They were 16/17-ish then. He married Crystal at 19 or 20, and they had a child approximately a year after that. Comics timelines being what they are, Luna has been 6 since like the mid-90s. I think the last time I bothered to work out Pietro’s age (which would have been during Dan Slott’s run on Mighty Avengers), I settled at 33-ish, just based on the other characters in his corner of the Marvel U, and their approximate ages.
Basically, he should be about 6 years younger than Steve is physically, and 3 years younger than Clint. Steve’s age is also stupidly ambiguous, and with Clint having his Brooklyn hipster phase, I have no idea what age they’re trying to project him at now. There’s also the annoying thing with female characters never being allowed to age, so Bobbi Morse (who should be a few years older than Clint) was mentioned as being 30 in like 2011. So there’s no good answer, but I’d put him (Pietro) somewhere in his mid-to-late 30’s, based on how old everyone else should be. Or in his mid-to-late 50’s, depending on how you feel about putting people in stasis as infants, I guess.