It's basically this.
At 16- they can sign a pro contract but for maximum of 3 years.
At 18- Whatever the feck they feel like.
No it's not…
At 15/16 they are offered a scholarship. The club usually uses a range of offers depending on how well they thing the player will progress. As most players don't make the grade many clubs are pretty conservative unless someone is clearly outstanding. This is sometimes hard to tell or the player is having trouble with injuries or other factors.
Here are a few examples:
One year Scholarship (1+0)
Two year Scholarship (2+0)
Three year Scholarship (3+0)
One year Scholarship plus one year Pro deal (1+1)
One year Scholarship plus two year Pro deal (1+2)
And so on…
When we sign a young talent from another club at 15 yo we might offer a 2+2 in a bid to attract the player, guarantee them a Pro deal to ward off competition from other clubs.
So as a further example, one player might be offered a one-year Scholarship and does OK in his first year. He might then be offered further one-year deal. He does OK and has now turned 17. We might still not offer a Pro contract because we are not certain he will make it. So he is offered a further one-year Scholarship. Other players are released after the first or second year because the club decides they won't make it.
Someone like Andreas Pereira might have been offered a 2+2 so he is on a Scholarship for two years, then signs a two-year Pro deal when he turns 17…the legal age. Which means he is out of contract when he turns 19.
Many agents suggest that young players sign short-term deals to start with. The club also prefers this, again to see how the players progress. The club also takes risks that the name of Manchester United, playing with great players, the youth history etc will entice youngsters to stay without committing to long contracts.
To be fair to the club, I wouldn't want everyone on five year deals…the backlog of players would be horrendous and most would need to go on loan to get games.
I understand that 90% of this seasons U/18's have been offered one-year Pro deals and most will probably sign it. However, on occasions, for a range of reasons, some players turn down the deal and move on.
It also comes down to the club finances and how much budget they have for youth contracts.
I don't know the specifics with James Wilson, but he signed as a Pro when he was 17 which suggests it was a two-year deal.