The most recent literature that I'm seeing is showing the most pronounced time of thymus involution occurring around puberty, suggesting a programmed switch from a growing to reproductive phase of life.
"Regardless of the seemingly crucial role of the thymus in preserving homeostasis, its involution in humans and other mammals begins in childhood and peaks around puberty, resulting in an almost completely non-functional organ in aging." https://www.sciencedirect.com/science/article/pii/S156816372...
In this light the following makes sense (stopping or suppressing the switch to a reproductive life). "Castrating rodents before puberty or reducing the levels of sex hormones [e.g., by using Lupron, which desensitizes the luteinizing hormone-releasing hormone (LHRH) receptors] can attenuate or markedly recover the involution process in aging mice."
A much older study with different markers shows something similar: https://www.pnas.org/doi/epdf/10.1073/pnas.070061597
For those not familiar with immunology, the thymus is what populates your immune system with brand new T cells with freshly selected T cell receptors.
This suggests interesting anti-ageing opportunities, such as artificial thymi to repopulate your immune system and keep a healthy and diverse repertoire.