The Gunners full-back is preparing to take in the summer tournament with Spain and he considers it to be an important stepping stone for those hoping to go on to enjoy further success with club and country.
Rashford, though, has been left out by England after becoming a regular in the senior fold, with Jose Mourinho having been vocal in his opposition to a U21 call during the 2016-17 campaign.
Bellerin said ahead of an even due to get underway on Friday: “It's not about age. If Marcus Rashford is ready for England then he should play.
“Sometimes taking it slowly, taking a player through the paces is the best way to keep him wanting more.
“If you're there from the beginning, what else can you achieve?”
Bellerin has pointed to the Spanish approach as an example of how picking the best possible squad will help to deliver success and ensure that those involved are better equipped to handle life at the very highest level in the future.
He added: “You can see so many players who people think could be in the first team, like Asensio at Real Madrid and Saul, who has played more than a 100 games for Atletico.
“It's a great approach because it humbles you. If you want to be in the first team you have to show how much you're worth at U21 level.
“In the first team there's a lot of great players. What player are you taking out?
“People like Juan Mata, David de Gea and Ander Herrera have won this competition and know what it's like. They've moved into the first team, winning the Euros and the World Cup.”
England open their Euro campaign against Sweden on Friday, while Spain will take in their first fixture against Macedonia on Saturday.