If you are asking about military, cadete = cadet.
Talking about sports, football for example in Spain there are different categories by age:
Senior Absoluto
Sub-19 Juvenil de tercer año
Sub-18 y Sub-17 Juvenil
Sub-16 y Sub-15 Cadete
Sub-14 y Sub-13 Infantil
Sub-12 y Sub-11 Alevín
Sub-10 y Sub-9 Benjamín
Sub-8 y Sub-7 Pre-Benjamín
So, if you are 16 and you play football you’ll be “cadete” . In UK it should be young player or Under 16.