would you sign this youth if you were me?

[Golden Boys FC]

How do you figure he will tire less than most? that would depend on his fitness skill and it should be the same as any other player of his quality, give or take a few points.

He will tire less than most because of his age (18)
If you PT an 18 year old you usually need to use 1-2 greens to recover him before his next PT (depending on his health starting point)
If you PT a 22 year old you usually need to use 2 greens
If you PT a 24 year old you usually need to use 3 greens

I'm only basing that on what I've witnessed during PT my players.