No matter what age you are, that age is new to you, and you learn surprising things about being that age. Cracked has a pictofacts post about things that surprised you about adulthood, and it's obvious that the top contributors range in age from new adult to newly elderly. Having to change your own sheets comes a long time before waking up with aches and pains every day.
However, thanks to sharing on the internet, we needn't be too surprised by things our parents didn't tell us about aging. No matter what (adult) age you are, there's something you can relate to in the list of 23 disappointing things about being an adult.