Anectdotal, but I was a lifeguard in my Highschool years nearly 2 decades ago. Scrawny at 5'9 140 lbs. However, we spent a lot of time training and practicing rescuing people. We made sure all our staff could individually rescue someone that is 250 lbs+ across the pool, tread water holding bricks over our heads and swim respectably fast, trained how to scan the pool so that there are overlapping eyes and no blind spots on each area. We were all CPR certified (however we deemed the lifeguarding a failure if you've already needed to resort to CPR due to drowning). We spent hours practicing loading a victim with a spinal injury in water onto a backboard (spinal backboard procedures), and even had EMS stop by to train us on first aid. Refreshers on procedures and training were done quarterly.
When not at the side of the pool, we're cleaning bathrooms. Good times for minimum wage. Even saved a couple of kids that wandered too far off into the deep end during summer. No one even noticed they were drowning.
I hope at least some of this provides you with some trust to your local lifeguards even if they are young.