I'm 30, my hair is pretty fine/thin. But it needs a lot of conditioner. I would like to try a serum or leave in product. It just can't be heavy, or my hair will look greasy. I'll attach a picture and Maybe that will help. :/
It's not a leave in one but Biolage makes a deep conditioning mask that comes in a pot. I really like it and I have baby fine hair. I highly recommend it!
I use Pro Naturals Hair Repair System that consists of shampoo, conditioner, hair mask and Moroccan Argan oil leave in treatment. It works well for me and restored my damaged hair. You can read all about it for yourself.