Certification

Well, of course not everything is suitable for all ages, specially animes, some that contains sexual materials, nudity and some do not, check the following for info about each Certification, mostly those are used in Anime DVDs. Please note that the following certification guide is different from each country, but here i have wrote for you the mostly used ones:

3+	Doesn't contain any sexual relations or anything for adults, it's For-All-Ages, but mostly for children. Well, here parents are suggested is their sons are still kids, so they must tell them that some scenes are not real and can't be happened in our real life.
7+	Some strong language, such as 'damn you' etc., you can rate that as 3+ sometimes or for all ages. Also parents here are suggested if their sons do not understand that this is an 'anime'/'manga' which's not a real thing.
13+	Some sexual relations, kisses, girls wearing so short dresses, but no nudity. but sometimes the series isn't totally 13+, some times it might reach 16+, but I don't think it reaches 18+. Parents are suggested if their sons haven't reached 13 or still not adults.
16+	This anime/manga series contains nudity and sexual relations, Parents are suggested for viewers under 16 years old.
18+	Hentai, this anime/manga series contains pornography... which called Hentai Anime/manga, or Echhi, and if love between boys called shounen ai, and if hardcore between boys called 'Yaoi'.