As long as you're doing nothing bad or sexual with your child when sleeping in the bed with them then really it's not inappropriate at any age to sleep with your child.
A lot of families years ago had to share the same bed with each other and there's nothing wrong with that.
However usually by age 7 or 8 children will usually not want to get into the bed with their parents and that's usually when most people decide it's not right to sleep with their child.
But even at 7 or 8 and even 10 to 14 it's okay however most kids that age will no longer want to sleep with their parents.
As long as your kid wants to sleep with you and you want them to sleep with you as well then it's okay and nothing bad about it.
Most people do think by age 8 to 10 that it's getting inappropriate to sleep with their child but that's their opinion and I say it's not inappropriate as long as you're doing nothing else bad to your child and are just sleeping with them.