[18 months +] Teach your child to count through incidental counting here and there, when they have attention for it. This might be seeing a whole lot of balls in a book, once you’ve named ‘balls’, if they are still looking, you could name the colours, sizes or count them. They will need to hear numbers and see you pointing as you count many times before they will get the concept.
[2 years +] Once your child is getting closer to being more interested in counting, start asking ‘How many…?’ before you start counting. Although some children can count, I see many who also have no idea when I ask ‘how many…?’. They need to learn that the words ‘how many..?’ and ‘count’ mean to start counting!
Don’t worry, these are the earliest ages you would worry about beginning counting. Go along with your child’s attention for it. You can always model other vocabulary instead.