To check-in at the hotel you must be of age and therefore be at least 18. Of course, if you are travelling with children, there won't be any problems as long as they have their own identity card or passport.
Please note that some accommodations may be marked as "Adults Only" or reserved for adults only and therefore children will not be allowed access.
Generally the minimum age allowed varies according to the hotel policy: from 16 to 18 years, although in some cases it could be up to 21.
When booking, always check if the accommodation you're looking at is indicated as "Adults Only" and the minimum age allowed for check-in.