Strange but realistic.
Well, it depends on the level of responsibility you have. We live in a generation where age is no longer defined by the number of years, but defined by one's ability and capability, which in turn increases our responsibility. Knowledge is what defines age, that's why God can bring something out of nothing.
Looking at the story of David in the bible, he conquered a man, called Goliath. Goliath was literarily 10 times David's age, and so is his stature. What do you think made David to make such a sophisticated choice/decision? His Knowledge. He has grown in knowledge and wisdom, but his physical strength just seems abstract. Don't forget! God was also with him. [ Job 12:12
"Wisdom is with aged men, With long life is understanding"]
"I'm 23, but why do I feel 100?" also depends on what you've been exposed to, which could be of good or bad. Our experiences in life increases as time passes by, but it's either this experience adds to us or even takes away from the little we have. But, the bible says in James 1:2 "Count it all joy, my brothers, when you meet trials of various kinds."
Our challenges sometimes can make us feel old at a young age, but just understand and believe that its just for a little while. God Bless You!