Depends on his other stats, eg:
http://www.battrick.org/nl/playerDetails.asp?playerID=1080923Stamina: competent Wicket Keeping: woeful
Batting: worthless Concentration: woeful
Bowling: strong Consistency: respectable
Fielding: respectable
He's currently going for > 2 mill
I would say if you have lots of money already then keep him.
If you don't and you're going down this season then probably sell him, get a new trainee to replace him and he'll be a good player for you in div 6 next season and 5 the season after. You can then use the interest earned from your cash to develop more quickly.
I'm keeping my 18yos, but I did just sell a 20yo for just under 1 mill, so will easily be able to build a cash pile and also reckon my season will be ok with a top 4 finish - although without the 18yos (my 2 best bats) I'd be pretty screwed!
edit: If I was going to sell I would set a high asking price though to ensure he didn't go cheaply (you'd be better off keeping him in that case).
I got a fielding pop today on my 17yo all rounder, which is nce as it means his fielding isn't too far behind where I want it to be.