Haven holiday parks, booking under 21 years of age...?

Me and some friends tried to book a holiday with haven today via thomas cook but when the agent who was helping us phoned Haven they said because we were not all 21+ they wouldn't accept us because they're a 'family holiday park' but when we looked in the brochure in the small print it says "you must be 21+ to book" it doesn't say anything about having to be 21 to actualy stay at the sites. Because the few of us that went to thomas cook today were only 18 and 19 we obviously wern't allowed to book but if we got somebody 21 to book for us could they turn us away? I don't think they could because they allow children in their parks, has anybody else had this problem?