Your impression is misguided. It is an enlistment incentive to go straight to SEAL training. Army Special Forces and Rangers have similars program. Men can become Green Berets or Navy SEALs before they can even legally purchase alcohol. Or a handgun.
I was watching a documentary about BUDs training and one of the people that made it through was 18. However you still have months of training after BUDs before you become a SEAL. But I would assume a 19 year old SEAL is perfectly realistic.
FWIW, my understanding is if you want to join one of the counter terror military units (delta force, devgru, etc) you have to show excellence in one of the special forces (SEALs, rangers, green berets, marine recon, etc). Most members of seal team six (devgru) are in their late 20s and early 30s, I think you need 5+ years as a SEAL before you can try out to join seal team six.
Do you think you are confusing the two (special forces units vs counterterrorism units)?
In 1967 at Boot Camp in San Diego we were marched over to the auditorium and watched a couple movies about Navy Seals and their training. When they were finish they asked for volunteers . Those who volunteered were pulled from the company and went through a 4 day indoctrination of physical training , written tests. They were returned to their companies to finish Boot Camp . The ones who passed the indoc. went to Coronado for BUDS training. My Recruit RCPO became a Navy Seal and was killed in Vietnam. I have a very close friend who served with Seal teem one and teem six in Vietnam . Yes you can finish BUDS and become a Seal in under 2 years. But for Seals the whole show is never ending , when you are not deployed you are in training You owe Uncle Sam 6 years to be a Seal. If you wash out you still owe 6 years. My friends son washed out of BUDS and ended up Unrated in the fleet and became a Cook. He was not a happy camper !!