I suspect that many teenagers are never really taught the importance of a proper warmup, or disregard it entirely, just to have some fun and not having any concerns of the horse as of a living, feeling creature. Those are bad work ethics in my world.
After my horse has been groomed and tacked up (I also groom with massaging movements to help his circulation), I take him for a quick lunging or groundwork session - just to see how is he moving and to check for anything that might be off. If everything seems alright, I mount and at first do a lap on loose reins, at an active walk. Afterwards, I take my time to flex him, go some circles and figure-eights, first at walk, then at working trot. After 20 minutes or so, we are ready to work, however, I always keep in mind that he might be stiffer one day or another, so it might take more or less time, depending on the circumstances. Also, I always warm up longer if the weather is colder or if heavier work is planned.
I have come a long way, to surrender my shadow to the shadow of my horse.