I had my baby in a swing at four months. It’s all how your baby can hold their head and body up. Every baby is different.
My boy is almost 6 months, I put him in a baby swing at 5 months as that’s when he was able to independently sit up, I always put a blanket behind him for extra comfort and to pad him out a bit x
I agree with @Vanny !
The book I'm reading said either 7 or 8 months!