I'm a guy in my mid 20's and have never had any issue getting an erection. However, when I was with my girlfriend a couple weeks back she was going to go down on me (this was the first time we had done that as we've only been dating for a short time, roughly a month or so). The night before I couldn't keep it down. When she started to go down on me I wasn't getting an erection even after several minutes. Embarrassing, needless to say. Yet the previous times we hung out and were just laying there cuddling/spooning I couldn't keep it down. I used to be able to get erect without issue. I'd even be just sitting at work and be like, well I guess I have an erection.