Is it possible to develop lack of attention with age?

I'm 20, and I feel like I'm going downhill. At the end of highschool, I received the highest possible mark, and everyone considered me extremely intelligent. I was dedicated and was always labelled "smart". Now I'm at uni, Ive failed two courses and am only just passing the program. I find it hard to sit down and study for any period of time.

When people ask me questions, especially at work, they have to ask me twice, because even though I know they are talking to me, I never really listen the first time.

My parents regularly tell me "It's never clear with you is it! I have to tell you every 5 minutes!".

If a conversation is uninteresting to me, I will unintentionally stop listening and think about something else, or "zone out" and when I finally do come around, I find that I have no idea what they are talking about and sometimes I don't even know what I was thinking about. I do this with lectures too. It happens more if I'm triggered by a word that leads me to think about something else.

I've also started leaving letters out of words when I handwrite, and often leave out the first letter and have to put it in last, as well as often getting letters mixed up in words. Not sure if it's related.

Is this just a bad case of not being present and lack of motivation? Or is it possible to develop an increase in lack of attention?