In the news

UK Marines are an elite group, whereas any military-age American can become a US Marine by having basic physical fitness, an IQ > 79, and signing on the dotted line.

Same as the Royal Marines then:

You don't need any qualifications to become a Royal Marines Commando
but must be aged 16 to 32. To be an officer, you'll need A levels or
above and must be aged 18 to 25. If you're under the age of 18, you'll
need the consent of a parent or guardian.

For the Royal Marines you have to be:

At least 15 years old and 9 months when you apply Over 16 years old on
entry Under 33 years old when you begin basic training

Royal Marine physical eligibility is no asthma, no broken bones and BMI below 28.