Absolutely ignorant statement, Gerard has played and excelled at every NT level for Spain...Even came close to being called to the WC for the senior team...Everyone in Spain knows what sort of quality the player has
It is "somewhat easier" to be a NT level when you are younger.
To enter a U-16 NT team, you need to be the best between boys aged 15 and 16 years.
To enter a U-18 NT team, you need to be the best between boys aged 16-17-18 years.
To enter a senior NT team, you have to be among 3-4-5 top forwards in your country among players aged 17-18-19-20-21-22-23-24-25-26-27-28-29-30-31-32-33-34-35 years.
The same is on the club level.
In youth ranks you have to "beat" only guys of your age.
In senior ranks, you "fight" with other guys (who were the best in their generation in the past, the same as you are now).
But you are "fighting" with 10-12-14-15 different generations.
So, shorter:
1. to enter a youth NT team, you have to be in top 3-4 forwards of your generation (plus, your all forward opponents/teammates are equally as unexperienced as you) = against 3-4 young players
2. to enter a senior NT team, you are fighting against 3-4 top forwards from 15 different generations (plus, guys from older generations also have a lot more experience on a club and NT level than you, so again they have some advantage over you currently)
= against 45-60 top players
Also, in youth NT teams, you are playing against 17-18 years old defenders.
And in a senior NT team, you are playing against 25-30-35 year old defenders.
This is why a lot of players are gems in youth NTs, and later they disappear.
There is a huge difference in quality between youth and senior club football and NT football.