Depends on age. Most men , as they get older, aren't able to get an erection as easily or have it last as long as they could in their early 20s. And certainly not at the same level of 16. I'm a woman. But this is what I've heard and also what I know about biology of reproductive system. It's totally normal to change. It's part of the body's maturation. Nothing for men to worry about or think there is something wrong with them. There isn't. Women's bodys change too. Also I'm sure most men are grateful it doesn't work like a 16 year olds their whole life. Imagine having to carry around a book or backpack all the time to hide an unwanted erection. Guys have told me these incidents were mortifying to them as teenagers.