What are the Jewish customs on sex education of children or teens? How is it done, and at what age?
Or is it completely avoided until marriage?
Are they taught what it means for example a man can not sleep with a man, Yehuda and Tamar...?
Books or sources on the subject?
From what I have heard from multiple Rabbonim in Baltimore, the 'birds and the bees' discussion used to be (and should ideally still be) given over from father to son, mother to daughter, when they recognize their child is beginning to notice changes in their body. For that conversation about the physical drives Hashem gives us, a healthy and safe relationship needs to be formed between parent and child. I remember Rav Yisroel Reisman saying he would purposefully learn Mishnayos with his sons at the right time about baal keri and krias shema/learning Torah which would naturally open up the conversation for him to discuss this idea.
Tragically many parents and children today don't necessarily have the same relationship or sensitive insight to feel comfortable giving these conversations. (I'm speaking as an educator, licensed clinician, and close talmid of a few Rabbonim who provide guidance for parents) To fill that gap, Rav Avi Landa worked closely with Gedolei Yisroel to create the "Kedusha Talk" series. He's a mashgiach and licensed counselor supported by the haskama of daas Torah in either opening the discussion himself for a small group or giving parents the tools to offer their own children guidance. There are helpful resources at the linked GYE site discussing the Kedusha Talk.
I know of two books which address the topic with sensitivity and respect for halacha
Let me summarize is in short:
The general Haredi perception of the purpose of the intimate relations is [still] procreation. Lately, especially abroad (out of Israel) it is slowly changed into a necessary "leisure" part of Shlom Bays. I'll mention this later on.
The approach to sex education is similar to other Mitzvos that are not studied or discussed until applied. Should I learn how to slaughter? I won't if it's not practical for me, so why learn relations without relations? Also Hilhos Ishus, Niddah etc. are only learned when applicable (for married men).
Besides the simple "closeness" (והאדם ידע את אשתו) between a man and a woman mentioned in the sources (Mikrah, Gemmorah etc), no details are discussed until marriage. Actually, this is pretty smart, because it is not only a waste of time but very impractical (not mentioning לא תתורו which is a clear prohibition). It provokes unnecessary curiosity and possible very serious transgressions of ז"ל, attempts of gaining personal experiences etc.
Sexual sins (ז"ל, משכב זכר, עריות) are freely mentioned within the learned material thru the Yeshivah years, but (as being pretty much self-explainable) never studied in details (like what exactly constitutes the עבירה and the likes).
[only] Prior to the wedding, the bride and the groom take a [short] private course with Madrich Chatanim/Kalot that explains what's done and how it's done. For most guys and [less for] girls it is a huge psychological shock, so usually it takes months up to a year to figure out the right intimacy (many frum guys can't function at all in the beginning), therefore this guidance continues into their marriage.
Lately, the awareness of women's needs grows even in Israeli Haredi community and the groom is instructed not only on the ways of mere procreating but also pleasuring his wife [not necessarily naturally] as I brought in #1. This leads to more and more Madrichim (and Rabbis) awareness and recommendations for the guys on using accessories for women (some Chassidic groups have their own private shops(!), but you need Rebbes approval to visit those).
Needless to say, as the access to media that lacked for thousands of years is becoming pretty ubiquitous, many of today's Yeshivah students have Internet and all the information they need in verbal and graphic forms. This situation is similar to what happens in the secular world but a couple of years later. This is a huge problem for the whole system and especially Rabbonim, but it is natural and needs to be dealt with.
Another issue is contraceptives. The Halachic standpoint is pretty clear hear, so it is practically only the "women's problem". However, as I stated, the awareness for non-procreative relations is going strong and it slowly becomes a standard with Haredi women also, not only as an excuse as it was perceived for years.
I personally have known many Jews that teach sex education in a mostly secular way to young children but then switch to a more halakhic/religious methodology when the children hit puberty. As a result of their influence I'm also planning on teaching my children in a similar way.
Some of these Jews gave us a good book recommendation for teaching sex education to toddlers that I feel is worth sharing. For those who may be interested in a completely secular sex education for young children I recommend the following book: https://www.amazon.com/What-Makes-Baby-Cory-Silverberg/dp/1609804856
The book teaches a dizzying amount of facts and presents them in a way that's easily accessible to children, while at the same time side-stepping the actual discussion about sex which you would then (hopefully) have at a later time. The book is solely focused on how babies are made and does not cover concepts such as sex, intimacy, homosexuality. But it does cover sperm, eggs, vaginal delivery, c-section, and miscarriage in a friendly and easily understandable way for a toddler to understand.
It can be jarring to picture teaching a 2 year old about sperm and eggs but we've found it to be very useful for our toddler. We're expecting another baby and our current child feels very caught up. Our daughter knows that the baby is growing in mom's uterus, and that the baby is made because I gave mom a sperm and mom gave an egg and we put them in the uterus and now the baby is growing.
This book is not geared toward teaching any cultural or religious norms, so that would still be up to the parents to do. The book is not a replacement for the very real and necessary teachings about sexuality and intimacy that is incumbent upon every parent to teach their child. For the teachings about sexuality and intimacy for teens/adults there are many great recommendations given in some of the other answers here.
