Teams don't want to play under 18s until they have appropriate protection in place, with robust contracts and such. If a player is late in adolescence, this is also a factor.
Not at all. Youth is basically being hoovered up and spat out all over the planet. It has little to do with being under 18, adolescence or contracts as to why they aren't playing first team football.
Having said that...
Tottenham's English current first team professional contracts (first signed)
Carter-Vickers, signed age 17 - loans 0
Walker, signed age 19 - loans 3
Sheffield United, Queens Park Rangers and Aston Villa
Rose, signed age 17 - loans 4
Watford. Peterborough United. Bristol City, Sunderland
Dier, signed age 20 - loans 0
Winks, signed age 16 - loans 0
Onomah, signed age 16 - loans 0
Alli, signed age 19 - loans 1
MK Dons
Kane, signed age 16 - loans 4
Leyton Orient, Millwall, Norwich City, Leicester City
You have to love what we are doing at our club!