do you lose braincells when you get older?

The short answer is yes.  But you don't have to wait until you get older, it's actually happening right now, and has been since you were 18 or so.  Brain cells do not regenerate, and we lose them at a slow rate over the course of our entire adult life.  Not to worry, though, as you have billions of them, and as we age, it is usually other disorders and issues with our brains that cause us to lose function or memory rather than cell loss.

One of the reasons stem cell research has been so intriguing is that it is one o the few treatments we have found over time that may be able to reverse diseases of the brain such as Alzheimer's.