Say a kid ran a 5K for the first time as a sophomore and ran 18:00. In two months could you get that kid to low 16s for sure? Let's say the kid's ultimate peak at age 24-26 would be low 14's, what type of shape could you get him in if you had 2-3 months starting from complete scratch?
You would still start them at lower volumes just like your other kids of similar age and experience. Say thats 30/40 mpw not 60mpw like you might have someone do running 16s and years of experience.
most important thing is to not get injured, an untrained and talented runner will respond pretty quickly to any training they do get as long as it's not too much volume. If you work up to 40ish mpw over 8 weeks or so as a base, and then get some good speedwork in during the season and peak properly I could see a runner like you describe going from 18:00 to low 16 in one season/4 months or so. But that would require everything to go pretty perfectly. More realistic would be a year.
I did this first season. Probably a high 17 before the first practice. around 16 at the end with some races slower and faster.
I ran on pure hate. Might recommend a more balanced approach.
I'm not sure I would call low-14:00s "amazingly talented". But if you just introduce some high intensity intervals and get them running a few miles every day you can definitely get someone into good shape fast. Exact numbers are very hard to say. But I went from a 19:xx equivalent over the summer my freshman year to low 17:00s by the end. So yeah, 18:00 to low 16:00s in 2-3 months seems reasonable for a young undertrained runner. Once again, wouldn't necessarily call low 16:00s "sweet shape" but yeah.