If there is continuity of personal identity, then we can say that people 'accrue' life, and so there's plausibly diminishing returns.
I'm not sure how this follows, even presuming continuity of personal identity.
If you were running a company, you might get diminishing returns in the number of workers if the extra workers would start to get in each other's way, or the amount of resources needed for administration increased at a faster-than-linear speed. Or if you were planting crops, you might get diminishing returns in the amount of fertilizer you used, since the plants simply could not use more than a certain amount of fertilizer effectively, and might even suffer from there being too much. But while there are various reasons for why you might get diminishing returns in different fields, I can't think of plausible reasons for why any such reason would apply to years of life. Extra years of life do not get in each other's way, and I'm not going to enjoy my 26th year of life less than my 20th simply because I've lived for a longer time.
I was thinking something along the lines that people will generally pick the very best things, ground projects, or whatever to do first, and so as they satisfy those they have to go on to not quite so awesome things, and so on. So although years per se don't 'get in each others way', how you spend them will.
Obviously lots of counterveiling concerns too (maybe you get wiser as you age so you can pick even more enjoyable things, etc.)
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.