How did Joe Biden become an senator at age 29?

I thought you have to be 30 years old to become a senator. Thanks for your input!

Public Comments

1. Senator Biden was 29 on Election Day 1972, but turned 30 before taking the oath of office for the Senate. He therefore was eligible to serve.
2. He was ELECTED to the senate at 29, but turned 30 prior to being sworn in (all that matters). The age for the House is 25, if I recall correctly.
3. So by age 33, he already had more experience in national politics than Obama....interesting perspective.