TL;DR
The thymus is the small organ that trains the body's T cells, and it shrinks with age. Two mouse studies this year point to drug-style ways of switching it back on. In one, a compound from diabetes research restored thymus function and structure in mice the equivalent of late middle age. In the other, temporarily easing the male-hormone brake on the thymus refilled the supply of fresh immune cells and helped cancer immunotherapy work better in middle-aged male mice. If either holds up in people, an aging immune system might be partly rebuilt instead of only propped up. For now, both are mouse results, one with very small groups, and there is no human trial.
Below the line, for paid subscribers: The full story: where it stands, how we got here and what's dated next; our Odds board on 4 milestones, starting with "The GW9508 thymus result is replicated with at least 12 mice per group by 2028"; and every source.