The syntax of a legal Internet host name was specified in RFC-952 [DNS:4]. One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit. Host software MUST support this more liberal syntax.
It's all ancient history, but before 3com domains with a leading digit 'just weren't done'. When they forced that through there was a general consternation on what crap internets are becoming.
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2.1 Host Names and Numbers
</quote>It's from 1989. ICANN was founded in 1998.