First Overnight

So maybe you experienced moms can tell me-- what age do you start letting your kids spend the night other places? Anna just turned 3 and we promised her for her 3rd birthday that she could stay the night with her aunt, whom she adores. Good or bad? Let me know what you think! She had a great time and behaved, so should we let her again sometime? Or is this too young? Every year we leave our kids and stay in a hotel for anniversary -- is that any different?