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Single letters are allocated to "root" DNS servers.

I built this a while back:

tl;dr: IANA reserved all the single-letter domains in 1993, except for some that had been grandfathered in.

All the two-letter .com domains are registered, and only available on the secondary market.

There's also single-character punycode domains, e.g. ¢.com which converts to http://xn--8a.com

x.com, but maybe it's not one of the oldest

There's x.org as well - the Open Group. But it wasn't registered until 1997.

Except that x.org was one of the single-letter domains grandfathered in 1993.