Children usually begin to recognize letters and their ABC's of the alphabet by age 2 years old.
If you get them a letter set such as a magnet letter set you can make it fun for the 2 year old to begin learning their ABC's.
Then you can teach them to spell things out such as Mom, Dad, Cow, Cat etc all those easier words which helps prepare a child for preschool and kindergarten.
By age 4 or 5 a child may know all their ABC's and begin learning to spell things much better and by 7 they should be spelling words out correctly with a lot of ease.
But some kids still have learning troubles and that's okay as not every child learns at the same pace as others.