Middle-aged is a term commonly used for people between 35 and 55. It generally coincides with the period in life when you might have children at home. In your youth, you don't have children. In your old age, your children have left home. However, a lot of people in their 30's don't like to be called middle-aged.

You could probably say "young adult" which is the best way to not offend anyone ;)

If they are specifically 30-40, another alternative is 'thirty-something'. e.g. 'There are quite a few thirty-somethings living in that neighbourhood.'