When it comes to any Star Trek, some fans will always stake their claim to who the "best" character from any series is. For Star Trek: Voyager, many would say that character is The Doctor. A hologram meant for short-term situations of a medical emergency variety, over time, becomes so much so. The Doctor as he was so affectionately named, became the breakout character of the show thanks in part to his actor, Robert Picardo's portrayal.
His evolution, growth, and dynamic actor gave The Doctor all the necessary skills (so to speak) to become the series best and most well-written character. At least in the eyes of many fans. So it wasn't a surprise that he'd be called back into action for the next live-action series, Star Trek: Starfleet Academy.
Yet, with the character returning to the franchise, we have to acknowledge that nearly 30 years have passed in real time from when Voyager first launched to now. Holograms don't age but actors like Picardro do. The famed thespian is now 70 years old, a far cry from how he looked all those years ago on the Voyager.
It's worth pointing out that this isn't the Doctor's first return to the franchise. Picardo returned as The Doctor in the second season of Star Trek: Prodigy. That show, however, is an animated show. Unlike Starfleet Academy, which will in fact be live-action. It's easy to make a piece of animation look younger, not so much with an actual person.
So, the question of change comes up. Should The Doctor have his appearance changed in the new show? You almost have to. Yet, how do you explain it away? When the god-like being Q returned to the show, he did so with some bad makeup and CGI effects. Since he didn't age, this made sense. Yet, he would then change his appearance to look older, so that his nemesis, Jean-Luc Picard, could see a contemporary of sorts.
The Doctor would have no reason to do any of that, as he's not a god-like being, an antagonist, or likely to run into anyone he previously knew nearly 1,000 years into the future. So if you were going to explain away the Doctor's aging, one would think a corporeal form would make the most sense.
You could make up some excuse about how he used "new" technology to build himself a body so that he wouldn't have to rely on holographic emitter on a ship or the mobile holographic emitter that he wore on his arm. A physical, living body would be far more likely to protect his program. Not only that but it would also be the easiest way to explain away why he's aged so much in the time since Voyager.
There are a few dozen ways you can explain the Doctor aging but this is the one that makes the most sense, while not burdening you with so many unnecessary plot points.