My son started hitting off the tee at eighteen months. Everyday I came home from work there would be balls all over the yard. But I never pushed him. The tee and his Little Tikes shopping cart of balls was his toy. At six he chose to hang out at his sisters's softball practices and hit off the machine and live pitching over tee ball. He thought tees were for kids who couldn't hit.
Once he started playing organized baseball at seven baseball ended in July. He didn't touch baseball equipment until the next March. There were other things to do and sports to play. It didn't harm his baseball development. Year round practice didn't occur until high school.