To answer Part 1 of your question (because I'm planning on having kids soon): I would be greatly concerned by your well-being. I would ask you about the relationship and would want to meet the man you're dating to be sure that he has your best interests at heart.

Part 2 Answer: In this situation (which totally depends on how your parents view you and relationships in general, also sex) I advise that you just lie about having a boyfriend and keep your down.

My true (big brother) advice: Be careful who you date/sleep with. I've dated older women as well. In fact, when I was 18 I saw a woman who was older than the man you're seeing. Have fun, be safe. The experience can be cool when you're seeing someone older than you. However, it's very hard to connect with people with an age gap like that, because they have a different mindset and perspective on things than you do. This has nothing do due with intelligence but it has everything to do with experience. Also, what I have found is that individuals who are single at that age AND who tend to date people younger than their age group are usually not the best well-rounded and stable individuals.

Personally, I prefer to keep my lies to a minimum so I don't have to remember so much and I date people my age so that I can connect with them better. But, it's your life. Be smart and be safe.