One thing to keep in mind with a kid's room is that it wont always look perfectly clean and tidy but you must remember the most important thing is that the room looks somewhat organized. What age is the perfect to get your kids involved in helping you clean their rooms? Any time after age 2. 

• Start young 
• Do it together with them
• Make it fun
• Make it a game
• Give them a chore they like

Kids ages 5-8: Getting Creative

• Limit the amount of stuff in their room (toys, books, etc...)
• Get some creative ideas on storage spaces.
• Give every item a zone/ place (books on bookshelves, toys in box toys, etc...)
• Make it accessible to them (put everything at arms length)
• Give them responsibility.  

Kids ages 9-12: Upping the responsibilities

• Teach them organizational life skills (put away books by subjects, etc...)
• Allow them to set their own routines
• Let them do their own laundry
• Let them choose/ make their own organizing tools/products
• STOP nagging them to do chorus