In British English, the noun watershed is used to denote the time in the evening after which programmes that are regarded as unsuitable for children are broadcast on television. The BBC Handbook of 1962 mentioned, against “the dangers of corruption by television through violence or through triviality, especially in the young”, the existence of

the BBC’s Code of Practice on Violence, its new 9.30 p.m. watershed policy, its intention to distinguish those programmes which it thinks unsuitable, and perhaps more important, suitable for children.

This use of watershed alludes to the literal sense of the word, which is: a ridge or stretch of high land dividing the areas drained by different rivers or river systems; the text in which is the earliest instance of watershed used as a television term that I have found indicated that the evening news served as the dividing line between family-oriented programmes and programmes suitable for, or aimed at, an adult audience; this text is Cereal Serials, by Mary Crozier, published in The Guardian (Manchester, Lancashire) of Friday 16^th February 1962:

Stories in instalments […] flourish on the BBC in addition to the normal steady output of single drama productions. They outflank ITV in its present embattled state. […]
Of the series, not serials, with a separate story in each chunk, the BBC’s Z Cars [see footnote] is now way out ahead of any possible rivals. Hard, fast, unsentimental, funny at times, and always gripping, it demands to be watched. In my blasé television heart it has taken the place of my Monday and my Maigret long ago.
ITV in its present state has little to set against these. Emergency—Ward 10 was a much healthier proposition than the BBC’s Dr Kildare, which (like The Defenders) continues to astonish by its blatant sensationalism, oozing sentimentality, and frequent unsuitability for that “safe for children” period before the watershed of the news. In recent numbers of these two series, I would cite the psychotic murderer and the suicidal pregnant wife.

The following is from the column Last night’s TV, by Richard Sear, published in the Daily Mirror (London) on Thursday 15^th November 1962:

If “Z Cars” [see footnote] is to deal in detail with sex and violence as it did last night, then it must be moved to a later time in the evening.
Otherwise, the 9.30 p.m. “watershed” between family viewing and no-punches-pulled TV which the BBC has rightly set up looks stupid.

Note: Z Cars was a drama series centred on day-to-day policing in the fictional town of Newtown, in Northern England; produced by the BBC, it ran from January 1962 to September 1978.