This is a brief summary about the work my colleagues and I have carried out over the last few years and about the implications the results of these and similar studies might have on our concepts of neuronal plasticity in the aging brain. We are interested in how brain morphology is altered in response to activity and functional challenges and we focus on adult hippocampal neurogenesis as a particularly interesting example of experience-dependent plasticity. The series of mouse experiments discussed here have led us to the conclusion that "activity" to some degree counteracts age-dependent decreases in adult hippocampal neurogenesis and that there are reasons to belief that a maintained flexible regulation of adult hippocampal neurogenesis is functionally beneficial. Thus, these data provide a novel idea about a cellular process that underlying the experience that "activity" is supportive for mental health.