If you could keep your body at a certain physical age...

30, 31 in a month.
I'd back it up and lock it at 26.
I don't think this requires TOO MUCH explanation

18 - I thought I was fat because waifs were in style, but my body was actually banging. I'm 72. I would most of all appreciate being arthritis free, with major organs all working at optimum.
Look after your knees, people.

23-27, that’s the age when I stopped feeling too young, but still had a serviceable amount of hair and could grow a good beard. Also I miss being horny. Late-thirties now.

Don't need the whole body, just turn all my joints/cartilage/soft tissue back to 18 and I'll just keep it moving (literally)

25, cause I started having arthritis symptoms at 26. I just wanna move my fingers fully again...
- Ouch, I'm so sorry you're having to deal with this!
  
  It's not actively hurting me, but I don't have full range of motion when grabbing things, my grip strength is way down, and it does hurt sometimes. It's like an 80% it's OK, 20% it sucks kinda bad type thing.

Old enough to do adult things. Body can process alcohol in a single night. Boner worked on command, not that it doesn't now but I feel like it was more so back then.

Age 20.
Wait. If the body stays at that age. What happens when the mind dies? Does the body still keep going? Blood keep pumping? What do we do with the bodies? Billions of bodies? Burn the bodies for power generation?

For me, I’d stay somewhere in my mid-30s. Not too young looking, but still able to do things. I’m currently in my mid-40s
- Same but 34 exactly, here. At 35 one of my wisdom teeth started acting up, and I didn't even become any wiser.

22, peak army physical fitness. After Iraq, my mental state made it very difficult to keep it up and that started the decline. I'm now 40, with back, feet, ankle and knee problems despite not being hugely overweight.

Probably 36, best shape of my life. I'm 43 now and it's not bad, gotta get back at it but I felt amazing in my body in my mid thirties.

I am not a follower of Islam, but I have read some of the materials, and they said that in heaven everyone will be 30 years old.
That seems like a pretty decent age. Like you're old enough to be an adult at 30, but you're still young enough to be able to enjoy life if you didn't have soul-crushing work to do.

I'd wanna go back to 22, currently almost 30. I developed a lot of health complications around 23-24, and 22 is the last time I remember my body not being in constant pain.

29, and I'm... uh... I stopped counting at forty.
At 29 I was done growing, used to my adult body, hadn't yet developed some of my current bad habits, and still had a youthful metabolism.

About 28 or 29. I'm currently mid 30s and starting to have a bit less energy than I used to.
- Felt like sometime after turning 30, something changed like a switch was flipped. Suddenly started gaining weight out of nowhere. Lol

I guess I'm the weird one choosing a much younger age. Perfect for me would be 16-18. Feel like that was when my body was most resilient.
- I was going to say 17. Adult sized with childlike resilience.

22 for me.
When I was around 22 I was in the best shape of my life. Assuming that the prompt means going back to the same state, I'd take it with no hesitation. It was before I damaged both my shoulders and started putting on weight, and it was before the chronic illness I have had shown itself and caused a lot more problems for me.
If it means that my body is reverted and not my mind, I could go to see my doctor and make sure that none of the damage happens, and be in a much better condition than I am now.

32, just before my metabolism slowed down, my muscles gave up and waved a little white flag, and without any warning at all, I was suddenly a fat bastard.
- You meant to type 32, but by putting a full stop after it makes it look like "1." on a lot of clients because of markup interpretation.
  
  Thanks, fixed!

Off the top of my head? 30. I was fully transitioned but my chronic illness has not taken off yet.
Currently 40

It's possible I haven't reached it yet, being in my late 20's. At least after the rest of my chest hair grew in sometime in the early 20's. Furry nipples on a bare chest looks really dumb.

I actually think it would be cool to be in your 70s - you can still do most things and can mostly think straight but other people think you cannot and so they don't bother pestering you.

As much as I'd like to say like 21, probably 25 since that's when your brain is apparently fully developed.

26 was the last age I didn't notice wrinkles and could grow a good beard. The beard keeps people from perceiving you as young.

19, because I started going bald very early. I'm about 40 now.
- /r/redlighttherapy may be able to help with that, even now, actually.
- That was less arbitrary than expected
  
  Even if I wasn't bald, I'd pick my early 20's. Adults generally don't get better looking as they age.

Late 20s. Full head of hair, putting on muscle happened almost accidentally etc.
I'm in better shape now but if I trained then like I have to now...
- Yeah, 21 because the hair, energy, lack of heartburn, ability to eat/drink anything all the time. I could run 10 miles averaging 6 min miles. Now my ankles and knees feel like I ran 10 miles daily for years on cement. (35)