We grow up and we observe and are taught the standard path in life. When in school we are taught we should go to university then we will get a job and in our 30's we should be having kids and getting married.
Many of us grow up thinking if you are in your 40's you should be settled down and if you haven't something wrong has happened. You shouldn't be doing X, Y and Z. You should be looking at retiring by 60 or so. That's when you will get to relax etc.
My point we live thinking if we do things away from the "norm" you are somehow not acting your age. If you are going out having fun with no major responsibilities in life in your 40's you are must be immature.
I don't really think that way and you only live once so for me I am 40 and in my head I feel 40. I don't feel any notion placed on me by society.
I expected to follow the path many do but it hasn't quite worked out that way. I am 40 and I am single with no kids and no intention to ever marry someone. For many that would mean them giving an answer like "I feel 20 in my head" but that isn't the case for me but I 100% understand if others thought that about me.