I worked at a dental hospital in the paediatric department as a dental nurse before I had DS and we always recomended introducing toothpaste when the child has learnt to spit out. Usually around the age of two but it varys (like everything) for each child. That is important so they don't swallow too much flouride, also the reason you use a low flouride/childs toothpaste. You definatly only need a smear, even for adults only a pea size amount. The toothpaste is only really there for the flouride and fresh breath. The cleaning is in the brushwork.