My son joined the Military on 9/9/19 my question is Do I claim him on my taxes since he lived with me until he left.

You can claim a child, relative, friend, or fiancé (etc.) as a dependent on your 2019

taxes as long as they meet the following requirements:

 

Qualifying child

• They're related to you.
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They're under the age of 19 (or 24 for full-time students).
  • No age limit for permanently and totally disabled children.
• They lived with you for more than half the year (exceptions apply).
• They didn't provide more than half of their own support for the year.

Qualifying relative

• They don't have to be related to you (despite the name).
• They aren't claimed as a dependent by someone else.
• They're a U.S. citizen, resident alien, national, or a Canadian or Mexican resident.
• They aren’t filing a joint return with their spouse.
• They lived with you the entire year.
• They made less than $4,200 in 2019.
• You provided more than half of their financial support.

 

When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them. There may be other tax benefits you can get when you claim a dependent.

what if they are a full-time student for half of the year before they entered the military?  so 5 of the 6 requirements are solid, but the 6th is not so clear.  They are full time students for 1/2 year not the full year and 19 years old at end of year. lived at home more than 1/2 year, made just over $6K, us citizen, etc. can still be claimed as dependent to parent?

Sounds like he is still a dependent since he was a student for at least 5 months and all the other tests are met.  Here is an extract from IRS Publication 501.  

 

"Student defined.

To qualify as a student, your child must be, during some part of each of any 5 calendar months of the year:

1. A full-time student at a school that has a regular teaching staff, course of study, and a regularly enrolled student body at the school; or

2. A student taking a full-time, on-farm training course given by a school described in (1), or by a state, county, or local government agency.

The 5 calendar months don't have to be consecutive.

Full-time student.

 

A full-time student is a student who is enrolled for the number of hours or courses the school considers to be full-time attendance.

School defined.

 

A school can be an elementary school, junior or senior high school, college, university, or technical, trade, or mechanical school. However, an on-the-job training course, correspondence school, or school offering courses only through the Internet doesn't count as a school."