The Beatles were all in their late teens and early twenties when the band properly started in 1960: John Lennon was 20, Paul McCartney was 18, George Harrison was 19, and their then-drummer Pete Best was 21.

How old were the Beatles when they started?

When people ask “how old were the Beatles when they started?” , they usually mean the ages of the members when the group first became “The Beatles” in Liverpool around 1960.

  • John Lennon: 20 years old.
  • Paul McCartney: 18 years old.
  • George Harrison: 19 years old.
  • Pete Best (original drummer, pre–Ringo): 21 years old.

At this point they were playing intense club gigs in Liverpool and Hamburg, working up the sound that would soon explode worldwide.

Before they were “The Beatles”

The story really starts earlier, with teenage skiffle and school bands in 1956–1958.

  • In 1956, John formed a skiffle group called the Quarrymen when he was about 16.
  • Paul joined John in 1957 at age 15, and George followed soon after, still only about 14.

So even though “The Beatles” as a named band dates from around 1960, the core trio had already been gigging together since their mid-teens.

When Ringo Starr joined

Ringo Starr, the drummer most people associate with the classic lineup, arrived a bit later.

  • The classic four-piece—John, Paul, George, and Ringo—came together in 1962.
  • Ringo Starr was 22 when he replaced Pete Best and completed the lineup that would conquer the charts.

By then, the others were still only in their early twenties, showing just how young they were when Beatlemania began to build.

TL;DR:
When the band really started as “The Beatles” in 1960, they were roughly 18–21 years old, with Lennon 20, McCartney 18, Harrison 19, and early drummer Pete Best 21.

Information gathered from public forums or data available on the internet and portrayed here.