As mentioned in my Sims 4 Recommended Mods list, I use MC Command Centre (by deaderpool) to set custom lifespans in my game.
I do this because I find the Normal lifespan too short for me, and the Long lifespan too long. I also have a particular issue with the Teen stage feeling much too long comparative to the other life stages, and the Elder stage far too short.
My current lifespan:
Below is the custom lifespan I currently have set. This is quite a bit longer than what I had previously used, but I wanted to enjoy more time with my Sims, and I also wanted to be able to have sufficient time at the new Infant stage. It’s essentially a midway point between the game’s Normal and Long options.
My previous lifespan:
This is the lifespan I used before, subsequently adjusted for the inclusion of Infants. It’s much shorter, and acts as more of a slight tuning of the default Normal lifespan (for the reasons mentioned above).