I trust Big Pete myself. The dude knows his stuff, or at least is good at giving the impression that he does.
I'm only proposing Ceri Evans because he's from my football-following lifespan. And I've followed the teams he's played for since his Nelson United national league debut. And I see him around at local games in Christchurch (his teenage son Oscar plays for my local club, Chatham Cup-holders Cashmere Technical) It's far from authorative!!! Could be someone younger but no one's provided one yet!!
But can rule out Brian Turner:
Brian Turner was 18 years 3 months old on his A-international debut:
Brian Turner (Wiki):Date of birth: July 1949
"He made his A-International debut in a 3-5 loss against trans-Tasman rival Australia on 5 November 1967"Don't know about Dave Taylor - can't find his birth date but he made his full debut as a sub vs. South Vietnam 10/11/67 in Saigon.
Another young 'un:
Chris Wood (Wiki):
Date of birth: 7 December 1991"made his debut in a friendly against Tanzania on 3 June 2009."
Wynton Rufer:
Date of birth: 29 December 1962
Full international debut: Merdeka tournament, Malaysia vs. Kuwait 16/10/80 (not vs Kuwait in the World Cup qualifier 1981 as is often supposed).So, aged 17 years nine and a half months.