I've had similar problems on my current save - I just always feel like I'm being ripped off when I spend that much on a teenager, no matter how high their potential.
Personally I'd wait since you'll probably get them cheaper aged around 17-18 when their contract might have fewer years to run, but granted that's a risk since you can't control their development and they may get injured or not want to move later on. I think £14m is too much to pay for any 14-year-old but it may end up being great business.
EDIT: just yesterday I very nearly bought a 17-year-old newgen from Real Madrid with my Stockport team for something like £41.5m:
- £10m up front
- £17m over 48 months
- £30k ish per game for about 30 games
- £7m after 1 international
- £7m after 50 league games
I pulled out though. £41.5m for a 17-year-old... I just couldn't justify it. Hopefully he'll be cheaper next year since he only has 18 months left on his contract.