I found that my short-term memory took a drastic dip after starting work. I don't think it's an age thing.
The reason probably comes from the fact that you at a point in life where you have much more responsibilities and the "tiny" things going on in your life to deal with. Thus, it becomes difficult to just commit things to memory rather than putting them down.
When I was still in college, my average number of items committed to short-term memory per day is probably less than half of what I have to juggle with now. With exception of memorizing for exams, I only have to remember what classes to go to, the occasional extracurricular activity, the once a month phone bill, etc. Now, I have client bills, personal bills, multiple reports, memos, stock portfolio management, personal finances, etc., competing for space with my other personal life details. So, it's not unusual to be absent minded.
So, don't fret too much. You're most likely normal.