Can you get a pulmonary embolism at age 18-22 from a blood clot?

Rare but happens. Pulmonary embolism is uncommon in young active patients, but certainly can occur (witness recent episode with serena williams). Risk factors include genetic predisposition (inherited predisposition to clot), injury to the leg, prolonged immobilzation (for example due to leg injury/surgery), pregnancy, and birth control pill.
Yes. Luckily, though, this is pretty unlikely. If you have a recurring blood clot problem, you should be checked by a doctor, you may need to be anticoagulated, that is, take blood thinning medication.