I feel like you're asking for the impossible... How would there be a place with young 20-somethings where you definitely won't get yourself in trouble at all?
I can't think of any place that would fit that requirement.
The "veggie" spas that twice recommended all have a tinge of shadiness with them (or perhaps a heavy dose of it), but they keep up a reasonable appearance of being on the straight and narrow. They would be your best bet to get most of your criteria (English is very uncommon though). Just ask the manager person that comes to you once you get seated in the lounge what you're looking for. Also, keep in mind that it'd likely be difficult to get in and out of those places in an hour. Closer to two hours is likely the minimum you should expect.
If your #1 priority is a serious and nothing-even-remotely sexual massage, I've been to Happy Foot (
http://www.happyfoot.hk/mavista/ ... ng-reflexology-spa) a few times without complaint, for both foot and body massage. It's a block south of the Central MTR.
Foot massages are given in a large communal room. Body massages are in a private room, but you wear a t-shirt and shorts that the spa provides. No oils or anything. There's no shower or spa, and there's a wide variety of masseuses, including aunties and dudes, but the occasional 30-ish woman. I assume the vast majority of them are not local but mainlanders, as they typically speak Mandarin amongst themselves while performing the foot massage. Every masseuse I've had can speak enough Canto, but English might be rare.
If you call them to make an appointment, the front desk person usually speaks good enough English and you can see if they'll honor your request if you have a preference on a masseuse. My typical request is for a strong massage and they usually deliver on that front, and they've asked before if I would be OK with a male masseur. Not sure what they would say if you asked for a young girl though...
I've been told the typical tip if you require a strong massage is ~50HKD per 50 minute session, although it is not strictly mandatory and I did give less my first few visits. I was gently reprimanded by a masseuse once later on and have gotten better service after adjusting.