The usual cutoff to start SEAL training is 28 years old, though waivers can sometimes allow older applicants on a case-by-case basis. For SEAL officers, the age window can be different, and one Navy FAQ says applicants must be commissioned before their 42nd birthday.

If you mean the oldest person ever to serve as a Navy SEAL, a widely cited example is Rudy Boesch, who reportedly served until age 62.