im 24 and i dont know how to swim too and I have the same problem,

can do freestyle but lack the skills to breathe but im slowly overcoming that problem, the solution

KEEP practicing swimming does build stamina hence it's just a matter of time.

As for those who totally dont know how to swim, can always start by holding the side of the pool and kicking legs while timing your breathing which is crucial.

1 funny method i found out was to cross your legs and just bob up and down the water and learn how to breathe in rhythm works for me though it's not the professional way to do it