Am I right in thinking that my boyfriend and I cannot catch STIs from each other during oral sex because neither of us have ever been sexually involved with anyone else?
Also, I know that sexual intercourse under the age of 16 is illegal but is other sexual contact illegal too?
While your risks for STI transmission may be lesser if both of you have not ever been sexually active in any way, it does not totally negate the risk. Not being "sexually involved" can mean lots of things for different people. Some people might only consider "sexually involved" to mean having vaginal intercourse with someone, and thus anything but that would make them not sexually involved. (Of course, in the clinical sense, the person is still sexually active if they are involved in any kind of sex.) Also, be aware that there are STIs that are transmitted via non-sexual means. Oral herpes, for example, is often picked up in childhood via sharing drinks, kisses from relatives, or other totally non-sexual means. The American Social Health Association notes that, "About 50 percent to 80 percent of the adult population in the United States has oral herpes (commonly called cold sores or fever blisters), with as many as 90% having the virus by age 50." And oral herpes can be transmitted to the genitals. So no, not having been sexually active before does not guarantee that you and your partner will be safe from all STIs. (While condom use will also not completely eliminate the change of passing on herpes, it does go a long way toward preventing.) Your best bet for being safe is for both of you to get regular STI testing (at least once a year) and to use condoms for all genital contact. You can find more information in the following links:
Regarding your second question, age of consent laws vary by country and state, so you will want to check out the regulations for your specific area. Generally speaking however, any sexual activity under the age of consent in a given area is not legal for you to participate in. This will include vaginal intercourse, oral sex, manual sex, anal sex, etc. Activities like holding hands or hugging that are generally not considered sexual in nature are usually ok. (Again though, consult the laws in your specific area if you have questions.)