At what age did you know if your child was right handed or left handed?

My son is 3 years 9 mos and does not yet show a hand preference. Even his preschool teachers and therapists comment on this. Some days he writes and colors with his right hand, other days with his left. It's the same way with eating, playing, throwing a ball, etc.

In contrast, my niece just turned 3 and is obviously left handed.

I'm wondering if my son's lack of a hand preference is due to his developmental issues or if this is common in other kids.