A lot of people have been talking about this so I thought I'd weigh in. No, he's not as old as Giffin/DeMatteis suggested, and mind that was without taking in time travel into account. If Booster was 35 then Lian Harper would have been a teenager when she died. My theory was that Booster was 25/26 because everyone tends to forget how young he was when he came back in time. He's NOT as old as Ted, Guy or any of the Silver Age heroes. I think because he hung out with Ted people assumed that. Booster was said to be 20 when he arrived, at least a year younger than Dick Grayson was. BUT there are also other things to take into account. In BG #1 VOL #2 Booster reveals his birthday is December 29 2442, his family was hounded by Broderick for months and he returns to the future around Christmas in 2462. Which means he was actually 19 when he went into the past and made a name for himself.
The time machine they used also miscalculated and put them a few months later than they planned. He skipped a head in 52, wore a suit that stopped his aging, etc. So while some thought that Dan Jurgens' age for Booster (23) was B.S. it works for me. For one thing Giffin and DeMatteis date everything making Max much older than he's supposed to be. Yes Jurgens' age for him was younger than I expected but he jumps ahead of time so it's not as big of a deal as it would be with other heroes. Jurgens already hinted at this in the Day of Death arc when Cyborg told Booster that he didn't look much older than Donna Troy, who I believe was 19 at the time since that's how old she was when she married Terry Long.
It's also the only answer that makes sense in regards to the "Michelle is clueless on how much she missed out on" plot. I refuse to believe Booster is more than a few years older than the last time Michelle saw him. Michelle isn't stupid enough to not notice that her brother aged 15 years since she saw him last. Which really annoyed me further since apparently this was obvious to every JLI member outside Max during G/D run. His teammates didn't even know him that well at that early stage of the league and this is his twin sister that he grew up with! She'd notice it before them!
And if you can believe Dick Grayson is still under 30 despite how long he's been around, and that Bruce Wayne is STILL in his 30s then it's really not that big of a stretch.