Its not really normal but it is more common than you can believe. I have had DDD since my early 20s and working to manage the pain for the past 10 years.
I've never done anything that has strained my back and I've never injured my back. It just seems odd that it could just happen to me in my early 20s.
i am 23 as well and i just found out today that i have it also so dont feel alone i guess you could say it happens to the best of us lol. i havent done alot to injure my back either but i found out mine was caused from abuse when i was younger so maybe something happened and you didnt even realise it