People consider Nketiah an Arsenal product. I think if you have a two or three year impact on a teenagers development it is fine to consider them a product of their time with you. People just need to understand that players spending time at more than one academy isn’t at all unusual anymore.
The criteria most commonly used for determining a youth player is:
1. Played in your Academy (you can hardly be regarded as a youth player if you didn't play in the youth team)
2. 17 years or younger (at 18 the FA regards players as adults and you can sign professionally at 17)
3. Not played first team football for another professional club (if you have already reached the first team you are regarded as already 'developed")
So for United, loads of players coming from school, Boys Clubs or other Academies fall into this category. If they come from other Academies then it is reasonable for both clubs to recognise the coaching given to the player. At United, we don't keep statistics of youth players until they play for the U/16's. The reason is to be consistent with players in past decades before the rules kept on changing. So someone who goes to our Academy but leaves at 10 or 12...we wouldn't add them to our list for records and statistical purposes.
Some players miss out being qualified as youth players because they don't meet all three criteria. Here are some examples:
Shawn Goater played in our Academy and didn't play for another professional club but he was already an adult.
Lee Sharpe played in our Academy, was 17 when he arrived but had already played first team football for Torquay United. Isak Hansen was the same as was Bojan Djordic.
The Da Silva twins arrived at 17, hadn't played professionally anywhere else but didn't actually play in any Academy team.
Of the 250 (or 254 depending on whose records you use) Academy graduates that have played for United...all of them meet all three criteria.