From a quick internet search it would appear that most children have the fine motor dexterity to tie their laces between the ages of four and six, but as with all skills different children will learn at different times. It is important to ensure your children learn this skill, even though alternatives are available as whilst velcro shoes and elastic "no lace" laces may be fun for your children they will need to learn how to tie their shoes eventually.
My previous post on Tying Shoelaces has a link to a site that has clear diagrams showing the different methods of shoe lace tying.
When teaching your child to tie their shoelaces there are several things that you can do to help. First, choose one method of lace tying, and teach it consistently. Make sure that everyone who may be trying to help your child learn this skill is demonstrating it in the same way otherwise they may get confused. When you are demonstrating the lace tying make sure that you and your child are side by side rather than opposite each other, that way he'll be able to copy your movements rather than mirror them. Lastly, remember to give your child lots of encouragement and praise and if they find it too difficult this time leave it for a few weeks and try again.
If you have any top tips for teaching shoe lace tying please let me know!