One of the puzzles of the series is pinpointing the age of Baxter and Barney. The series never gives an age, but there is that one bizarre comment Shredder makes of thinking of Baxter as a son. Considering that Shredder himself is supposed to be pretty young in this incarnation, as clearly seen whenever he doesn't have his mask, that is a very off-the-wall remark that probably shouldn't be taken seriously. Going by physical appearance alone, the Stockmans actually look like they could very well be older than Shredder. I highly doubt they're younger than the same age as him, at least. They certainly wouldn't be young enough for Baxter being considered a son to make sense. But for the skeptics, it's not an opinion based solely on physical appearance.
I discovered a very bizarre item for auction on eBay: a fact file card for Baxter that was likely originally sold in the U.K. due to its "Hero Turtles" branding. By the way, Baxter is depicted with red hair on the card, meaning they actually drew Barney instead. Heh. So right away we have a contradiction with the series. Of course, while I find fact-file cards fun, I rarely ever consider them canon even if an official source has put them out. My experience has shown me that even official material doesn't always keep the facts consistent with what's shown in the series. Also, they're usually intended as tongue-in-cheek, joke material and not meant to be taken seriously. This card is no exception; it's filled with silly puns and jokes based around Baxter's tragic fly state, as well as totally random things. Baxter loves Country/Western music? He thinks he looks like Tom Cruise? Uh ... okay, someone was feeling really silly when they were tasked with writing up that card.
According to the card, Baxter is 3 feet tall and weighs 65 pounds. He's short, but I really don't think he's that short. Lionsgate's fact-file on the season 5 DVD set also can't fully be taken as absolute canon, but 5'2" and 135 pounds make a lot more sense and I go with those statistics instead.
Back to the question of age, another of the "facts" on this old U.K. card states that Baxter is "About twenty." That could possibly make Shredder's "son" comment make sense if you squint, but it makes zero sense when you consider that Baxter is Doctor Baxter Stockman. He has a doctorate. As bizarre as Teenage Mutant Ninja Turtles 1987 got, it doesn't exist in the world of Doogie Howser, M.D. Someone who is "about twenty" is not going to get a doctorate. They still have a long way to go before they can achieve that degree.
Of course, Turtles 1987 contradicts itself a lot. Contrary to the "son" comment, season 4's Raphael Knocks 'Em Dead gives us Baxter's twin brother Barney referring to Raphael as "young man." About twenty? I don't think so.
Given the assorted evidence, facts, and the presence of logic, I would be more inclined to consider Barney's comment the canon one and throw out Shredder's "son" comment. And I would place the brothers as older than Shredder. For my stories, they are in their early forties.