Wikipedia describes the West Side Story main characters as
In New York City in 1957, two teenage gangs compete for control on the Upper West Side.
for the 1961 film and
In 1957, the Jets, a gang of white youths, brawl with the Sharks, a gang of Puerto Rican youths, for control of San Juan Hill on Manhattan's Upper West Side.
for the 2021 remake.
So "teenage" and "youths". In the film itself there are few references such as
"[...] down goes a teen-age hoodlum."
But do we have any way to know their ages?