PhD: admitted with TA with 5h a week, is it standard?
"Contact hours" is standard jargon in academia for formal class time: hours when the instructor/TA and all the students are scheduled to be together.

So this basically means you will teach 5 hours of classes or discussion/recitation sections per week.  This does not count the time you will need to spend preparing your material for those classes, grading homework or exams, holding office hours, working in tutoring labs, responding to student questions via email or online forum, meeting with the instructor or course coordinator, and so on. (Although you are in "contact" with students during office hours or in a tutoring lab, the term "contact hours" doesn't include informal face-to-face time like this.)

Once all this is added up, I'd guess it will be around the 20 hours per week that you expected.  

This does not seem to be a special offer.  I would guess it's the normal offer that all the accepted grad students receive.  (In any case, decisions to make someone a "better" offer, e.g. less work for the same money, would normally not take into account their age or marital status - most universities would consider that inappropriate or even illegal.  Such decisions would instead be based on academic merit and potential.)

As you consider this offer, you'll want to find out what's actually expected of TAs in this program, and how much time and energy TAs typically have to devote to their duties.  Most math departments invite accepted grad students to the campus for a 1-2 day visit to help you explore what they have to offer.  This would be a good time to ask about TA duties - ask the graduate coordinator about the official duties, and ask the current grad students what they actually experience.  Sometimes there is a difference between the two.