My son got his first 2 wheel bike a little before he turned 2. At 3, he took his training wheels off, with a little help from his older sister. No one taught him how to ride with out, them he just did it. Now at 4 he has a 20" bike.
My older girls were about 4 when they got theirs, and 5 and 6 when they were riding with out training wheels.