At what age should girls be covered with Hijab?

There has been some debate between some friends and myself about at what age should parents start covering their daughters with hijab or niqab? Should it be from birth? when she starts school (mixing with boys)? At an age when potential husbands are being searched? when she starts puberty.... or should we simply speak with her at young age and explain that Allah desires females cover themselves and let her decide whether she would like to fulfill Allah's wishes or go against his will? Thank you all for your responses.