I thought it was only if you were eligible for the other country when you played for the first country at youth level.
Yep this. He played youth competitive matches for the Netherlands and wasn't eligible for NZ at the time so he can never represent NZ, however he was eligible for Curacao at the time hence why he can still play for them.
No it's not. There have been player who have represented different youth countries and go on to represent different senior country. Roly has not played a competitive match for curaçao therefore in three years time role can play for NZ
Yes there are players who have represented one country at youth level then represented another - but those players all were already eligible to play for the 2nd country at the time they played a competitive match for the original country. Eg Rory Fallon played competitive matches for England at youth level but was eligible to play for NZ at the time, hence was able to play for NZ at a later date.