For being loyal for all the time in helping Hagrid, working for the Lilin and making sure everything went smoothly and great patience, among other things – apart from being made Headmistress - McGonagall was granted to turn back the body clock to whatever she wanted to. The other part of her reward was she would be able to have a working womb – she was born with a uterus that had her unable to bear children, which was part of why she took up teaching. A way to be among children, even they were not hers and they were in later in life’s milestones.