"When you're 3 years old another year will be adding a quarter to your life so it of course feels a lot longer than another year in your 40s."
Adding a quarter to the life of a 3 year old, (36 month old) will make that person 3 years and 9 months.
Here's another example, using even #'s:
If I have ten apples and get 50% more apples, I then have 15 apples.
If I then eat 1/3 of those apples, (5 of them), I have 10 apples.
Maybe it's different in the metric system.
I'm in my seventh decade on Earth. I know things.