I mean, you say Nigeria aren't a football powerhouse, but football is their national sport. And more importantly it's the sport that the kids play growing up.Nigeria has won the under-17 World Cup 5 times and finished second three times as well in only 12 attempts. By far the most successful nation in that tournament. Brazil is number two, with their 17 attempts resulting in 4wins/6finals.
If they ever brought along anyone too old or not I don't know, but it is a bit suspect to me that they reach the final more often than not in a sample size of 12.
Yes, I know they have got very many to choose from but still, they are not a football powerhouse.
List of countries by population and their national sports (or "the sports the kids play")
1. China - Various (Table Tennis, Badminton, etc)
2. India - Cricket
3. USA - Various.
4. Indonesia - Badminton
5. Pakistan - Cricket, Hockey , Soccer
6. Nigeria - Soccer
I can kind of believe that Indonesia can field very strong teams at U17 level.
Plus like you said maybe they're a bit older sometimes