Having a stuffed animal and imaginary friends are two completely different things.
An imaginary friend (one you think is real) is a way children learn and develop into social beings, you can read a lot about children and friends like these. There are entire Psychology courses on this topic.
Having a teddy bear/stuffed animal is usually for comfort/company when you’re sick or tired or just need something fuzzy to cuddle. They make good pillows and provide comfort when you need a good cry.There’s nothing immature about that.