There are at least two places in Brave New World where you can find this. In general, the answer is that the adults expose the children to death early and often. They also make sure that the children associate death with positive things so they will not fear death.
The first place we see death conditioning is in Chapter 11. This is where the phrase “as a matter of course” is found. Dr. Gaffney tells John that
Death conditioning begins at eighteen months. Every tot spends two mornings a week in a Hospital for the Dying. All the best toys are kept there, and they get chocolate cream on death days. They learn to take dying as a matter of course."
What this shows us is that the adults are trying to have the children associate death with positive things. The children get to play with the “best toys” and they get special treats. This helps them to see dying as a positive thing.
We see the death conditioning again a little bit in Chapter 14. This is where Linda dies. As she is dying, there are eight-year old boys in the hospital. We are told that
They swarmed between the beds, clambered over, crawled under, peeped into the television boxes, made faces at the patients.
As we can see, the death conditioning continues through processes like these. The children come to see death as no big deal and they have no special respect or any sort of feelings for those who are dying. They have truly come to see death as a “matter of course.”
it begins at 18 months