How difficult is it to join the military with a family?

Yes you can do it. Entry Level training isn't filled with only single folks. It isn't easy but it also isn't impossible. After entry level training at bootcamp and AIT you'll head to your duty station where you'll bring your family. Speak to a recruiter and they will be able to answer all of your questions.

First off, I was 27 when I went to Infantry One Station Unit training, so don't sweat your age. There are some Military Occupational Specialties that are exceptions to this following statement, but you will attend Basic Combat Training and Advanced Individual Training on Temporary Duty Enroute status and will not be able to bring your family until you get your first duty station, assuming it is one that allows them. That is most place, except some places like Korea.
Yes, you get paid at Basic Combat Training. Your recruiter should tell you what paper work you need to get your family covered by Tricare (military insurance for the families) and you will get Basic Allowance Housing with Dependents while you are in Basic and AIT. The rate will depend on the zip code of your current location. After you hit your first assignment, you will either get Base Housing or Basic Allowance Housing for the zip code of your duty station. You will also get a Basic Allowance for Subsistance if you aren't in the barracks and have access to the mess facilities.

The only big obstacle would be your choice of branch. You might have a better chance with the Army. The Air Force is very picky when it comes to folks already with a family.