Some children carry on forming letters incorrectly for some years. How come? Maybe because the beady eye of the classroom teacher has not managed to actually see the child form each letter. And somehow the child has escaped the teaching that would have taken place. By escaped, I mean that they had a go, couldn't do it correctly, made up their own version and somehow continued to print their own version.
Some children find learning to form letters difficult. Some children may have difficulty with motor planning, with organising their fine motor movements and with remembering the movements. It can be very confusing for a child who doesn't learn from the visual model. Some children find certain capital letters easier to form, such as capital E which has straight lines only, and they may continue to use capital E's even within other small letters. Some children have difficulty forming letters that require changing direction, such a lower case e. As well, there are many other factors to learn........... where to start( different for different letters), direction to go, pressure to use, where to stop, how to plan ahead so the letter sits on the line. Some children put the speed on and sometimes this works, as the brain just does it without them consciously having control.
Young children often experiment with printing when they try to copy people around them, and this is to be encouraged. But when they are seriously interested then please teach them the correct formation. It's a kindness to them, because once they have practised it over and over the incorrect way, it can be difficult to unlearn. And then there is the question of capital or lower case? Many signs around town are in capitals. However, when using printing in useful ways......writing stories, writing letters, writing notes, etc, we use lower case letters with capitals in the appropriate place. So maybe it's like learning two languages.....teach both at the same time.
Some children do not learn from seeing the finished model, or from seeing the letter being formed. They may benefit from you holding their hand as they print it until they remember the motor plan. And it can be confusing to try and learn the motor plan of different letters one after the other. So practise one letter repeatedly for a while.