Answer: You may accept the prospective tenant. Under federal law, at least 80% of the spaces must be occupied by at least one tenant age 55 or over. The remaining 20% was originally reserved as "padding" in the event that one or more of the spaces became occupied by someone under 55, say, through death of the qualifying tenant and inheritance by a nonqualifying tenant. It use to be that the federal regulations suggested that intentionally renting some of the remaining 20% to nonqualifying tenants jeopardized the park's 55+ exemption, since it failed to demonstrate that the owner "intended" to hold the park out as a 55+ facility. That interpretation has been changed, and now landlords may rent out some of the remaining spaces to persons under 55 years of age.
However, remember that if you fall below the minimum 80%, the 55+ exemption will be automatically lost and the park will automatically become a family park - meaning that you will not be immune from a fair housing claim if you refuse to rent a space to a tenant with children.