The immune system does decline; I know this because I've seen some elderly people at the gym exercising, and they're always talking about their ailments, doctor visits, medications, etc. I've also seen some videos of older people who have taken an age verification test. Let me explain: it's a system that measures your age according to your physical condition, not the age on your ID card, and they all come out with the same age as their ID. This shows that they are still active, but their immune system is declining.