All interesting questions, some related real world observations: early last last year we took on 11 year old and 13 year old lads, neither with ringing parents, neither with any church connections and with the usual crop of sports activities for that age. The 13 year old is musical, and had self-taught himself guitar and piano. Both live within walking distance of the tower.
The 13 year old's mum stayed for practices, She seemed a bit over-protective, she told me he had ADHD but he just seemed like a normal 13 year old lad to me. The 11 year old's parents left him with us (he lives across the road from the tower). Both took instruction well and "got the idea" without any major issues, quickly understanding the relationship between what they did and what the bell did, and were able to act on that without prompting - they would mostly tell me what the problem was when there was one. I think that's perhaps a better indicator than how long it takes to physically master bell control? Both rang for the Coronation (assisted) and even ended up on the back cover of RW.
After about 6 months the 13 year old dropped out, no real explanation of why why but I didn't push it. The 11 year old is still with us and is ringing unaided, with a very nice style considering he's not a big lad and is on 2 big boxes. It's very clear he's "ringing for him" whereas I always had a suspicion the one that dropped out was being pushed a bit. The 11 year old's mum said he'd heard the bells since he was a baby and had always been fascinated by them, and was dead keen when he realised there was an opportunity for him to learn to ring them.
I don't regret the time teaching the 13 year old, one of our band's mainstays rang for around a year when she was a teen and came back to it 40 years later, so I'm viewing it as a long term speculative investment. In the case of our adult returner, it was clear from the start that despite the big time gap the basics were still in there, she knew when it "felt wrong" would say so and just needed an explanation of what she needed to do to fix things.
The fastest to pick up ringing in elapsed time has been the 62 year old recent retiree who started mid-November and rang his first call changes last week. But then again he's musical (cello, viola), extremely motivated and focused and has been getting an hour every week of 1:1 tuition - so perhaps no big surprise he's progressed so quickly. We have another adult who you is also keen, but due to family & work commitments can't put in as much time - but when I offered to do 1:1 sessions that fit in with her other commitments she jumped at the offer, so I'm sure will do just fine, even if it takes a bit longer overall - as she says "It's not a race".
Quite what you draw from all that isn't at all clear to me. Maybe musical ability is a good predictor - but then again, the musical teen didn't stick at it. Perhaps the best predictors are if someone quickly gets "grabbed by ringing" and also has the ability to stick at things? Possibly the biggest contribution from having a musical background is the understanding that learning ringing, just like learning a traditional musical instrument is a long process that takes dedication and lots and lots of practice? I think it's easy to forget just how hard learning to ring is and how much time needs to be invested. The recent retiree recruit has commented repeatedly on how much harder it has been than he expected.