Never too late to start BUT most of the effect will not be seen until next year if you are just now starting.
Yes antlers are still growing BUT the biggest opportunity to effect Antler growth is Jan - March before spring green up. Build their muscles back from the cold and rut= head start come spring on Antler growth
The farmer who was leasing my fields this year was unable to cut beans due to it being too wet. I've seen huge differences in deer weight and racks so far this year due to them having the beans jan-march of this year.