Legal age for pretty much anything in Equestria is 16, with marriage and joining the royal guard being exceptions at 18.  
And sure, we get drunk at times.
 
Im not aware of any tension between us. We get along just fine.  
After all, we are all in the service of Equestria together.
 
All guards are free to have relationships outside of the force, but it is heavily discouraged to have one between two guards. It tends to throw housing and scheduling out of whack, and theres also the problem of divided loyalty.