I'm 31, flexible (despite sedentary job), and lazy when it comes to warmups. If my 2 oldest kids are around (3 and 5) I do a set with each of them (probably a set of 25-30# and then a set of 40-50#) on my back, then go right into 135x5, 185x4,... If the kids aren't around I do some air squats, hold the bottom position, then straight into 135# for 5 or 10 reps.

I'm either going to hurt myself or I warm up very easily. If my shoulders feel inflexible I will do a set of dislocates with a broomstick before the 135# warmups.