Yes it's normal for them to leave stuff!
My friend is moving, her son has his own place, not lived at home for years, yet her loft still has some of his stuff.
My DD's both have stuff here, one still has a bedroom, even though she's lived in Scotland for years. My home is a haven to come back to when they need to. When one daughter split from her idiot husband, it only took half a day to put her bedroom back together from all of her stuff she had left behind lol
I left home 39 years ago, and my mother, who is almost 80 still has some bits of mine! And SHE has moved home twice!
So I don think it's unusual for us to still have our darling childrens stuff lingering (or lovingly stored) round the house, attic, loft, garage or even the garden shed.