In deference to my co-mod, we have discussed this. The scenarios you mentioned are of interest topic wise.
This is what we have agreed upon: Groping between similar age teens is acceptable. Sex between teens 15 to 17 only is acceptable, but not actively encouraged. Sex between an adult and anyone below 16 is strictly forbidden.
Sex is defined as the touching of genitals to another person, penetration or other wise.
Sexual Abuse is allowed to be mentioned, the trauma of which is allowed to be explored. The actual act, however, is not to be written in any graphic detail so as to elicit a sexual response.
As for the Hobbits. If they are old enough to have sex, then we can over look the age thing.