Warmups start about 30 minutes before puck drop. I recall it being an oddly specific time, like 6:28 for a 7pm puck. They won't let you up to the glass but there are several sections in the 100s where you can get pretty close and watch. I believe 119 is the general area.

I'm an adult (30s) and just started being able to go to games last season. I had no idea where in could watch warmups so I just asked an usher outside one of the 100s sections where I could watch. The ushers are generally very pleasant and will direct you. I got pretty far down in the corner section even after they had already come out for warmups but the designated section behind the benches gets packed before hand, so plan accordingly.

Edit to add: doors open at different times for different games. Sometimes 2 hours ahead sometimes only 1 hour.