How do I get a boarding pass when I'm connecting on another airline?

I'm flying from Atlanta to New York-LaGuardia, and then to Syracuse, NY on the 26th. I'm flying on Frontier Airlines up to LGA, and then Delta to SYR. I know that if I was flying on the same airline for both flights that I could just print both my boarding passes at ATL, but since I'm switching airlines, how do I get my second one to fly to Syracuse? Do I have to get in line again at the ticket counter at LGA, or can I just have one printed off at the Delta gate?

What age is considered an adult on airlines?

My husband and I are looking up flight tickets for my step-son to come next month for Christmas. I'm wondering what the age is that "considers" them an "adult".

Also, any recommendations on airlines? I've only flown once in my life haha.

Added (1). I should probably mention that he will be flying as an unaccompanied minor. He's old enough to fly as an unaccompanied minor but he might be at the age where they consider him an "adult". We know there are additional fees for an unaccompanied minor as well. This is my first time doing this so I'm really not sure where to look. TIA

Added (2). Thank you all! That's about the ages I was finding online as well.