I really don't think relationships stunts an individual's growth. Maybe only for those who identify themself through the person they're dating but people who do that have more problems than maturity stunted by a relationship.
I would also like to know who "they" is. cough*James*cough
When you think about it, it was only really our generation that people wait until they are 25-35 to get married. People were getting married at 18 years old. I know that my grandmother was married at 18 and my grandfather was her one serious relationship. They've been married for over 50 years and I wouldn't call them "immature" or say that a relationship stunted their growth as a person.
Anyway, I have spent much of my time single. It's only been recently that I've had a long term relationship. Let's see my relationship list:
Dated for 1 month
Single for 8 months
Dated 3 months
Single 1 year
Dated 1 year
Single 14 months
Dated 3 years and still presently in this relationship