Use it on a fighter you know you're gonna keep long term (because he's good, not because you like his name/avatar). If he shows some good hidden eg granite chin on his tale of the tape, you know he's got potential. Doesn't matter how old he is, although 18 yr old creations have the best future career ahead of them, with most time to grow and their fastest learning speed for longer. After 20 or 21 learning speed slows down.
It's a personal choice issue but I would use it on a young guy, specifically on my ground build guy's as intelligence seems to be weighted more towards that style of fighter. When you use it is largely irrelevant however it's worth saying the result from testing the same fighter twice can differential 10 points either away (I think) so its not 100%.
thanks for the input guys, greatly appreciated