It is likely that age limits on sites, for mobile homes, caravans and tents are determined by autonomous communities and not the national government.
Spain is divided in 17 parts called autonomous communities. Autonomous means that each of these autonomous communities has its own Executive Power, its own Legislative Power and its own Judicial Power.
If you are non resident in Spain you may still be liable for non resident Imputed Income Tax. It is the assessed value local Tax Authorities give to a property. In the case of mobile homes, caravans and tents it is determined by the number of berths.