Thymus atrophy and T cells with 40 year lifespans is the easy answer.
Yet residual or enlarged thymus is seen as a potential cause for autoimmune conditions like Myasthenia Gravis. And it’s often removed in these cases. So is it really good to have a thymus after 30 or not?
Thymus involution can be reversed in humans (see TRIIM and TRIIM-X trials by Gregory Fahy).
It seems that this intervention has a moderate systemic rejuvenating effect, too.
I actually suffer from Autoimmune issues, i.e. Rheumatoid Arthritis. Which I could have known about that Trial, few years back though given my luck, I'd probably not be accepted into the Trial. Still, it's good to know that there might be some hope in the horizon. My Rheumatologist says there is no real cure in the meantime, so taking pills is the only solution, unfortunately.