How long have computers been around?
This really depends on what you mean by "computer". Probably the earliest design for a programmable computer was by Charles Babbage in 1835. The earliest electronic computers were developed during World War II (around 1943) to decode German messages. The earliest personal computer - designed to be used by one person - was the PDP-8 was first released in 1965.
The first computer which contains most of the components of a modern personal computer was the Xerox Alto, developed in 1973 - it had a windowed, mouse driven, WYSIWYG operating system as well as a laser printer.
historically, the byte was whatever the computer said it was. There have been computers that use bytes anywhere from 5 to 36 bits long. Today however, almost all computers use 8 bit bytes. When there is absolutely no room for misunderstanding, the term octet is used instead, which always means 8 bits.