>>9990255 (LB)
Bill Gates is 64 by now, so the year when he reached the age of 13, would have been 1969. There were no microcomputers back in 1969, as the CPU had not been invented yet. If Gates indeed wrote his first program in 1969, he could only have done so on a minicomputer or mainframe.
According to [1], the Lakeside School Seattle, where Gates enrolled at the Age of 13, got provided access through a "Seattle computer company" to a timesharing system. Gates supposedly wrote a BASIC (Invented in 1963 at the Dartmouth College) program to play Tic-Tac-Toe.
However, according to [2], Gates enrolled at Lakeside School in 1967 (Age 11) and the School asked him to create a computerized schedule of classes. According to [3], the Lakeside Programmers Group got free computer time on various computers in exchange for writing computer programs.
Source [4] reports that "several faculty members" of Lakeside School acquired a Terminal, which seems to corroborate the assumption that the School gave access to a mainframe or minicomputer.
Most likely, the mainframe(s) or minicomputer(s) accessed by Gates were hosted in an educational setting, as these machines were insanely expensive at that time, and not widely available.
Source [5] reports that some of the first computers used at University of Washington were an IBM 709 and an IBM 650. In 1966, a Burroughs B5500 was installed, which offered dialup connections for Model 33 TTY devices at 110 baud. By the end of the 1960s, UW had only 5 computers installed: IBM1130, IBM1401, IBM7040/7094 and the Burroughs B5500.
The BASIC computer language invented at Dartmouth initially ran on a General Electric GE-225 computer in 1964. Beginning with an NFS grant in 1967, Dartmouth began placing terminals in off-campus locations. By that time, the GE-225 mainframe was replaced with a new and faster model, the GE-635 [6].
It is thus plausible that the Lakeside School got one of Dartmouth's off-campus terminals, through which Gates could have accessed a GE635 mainframe running Dartmouth BASIC.
[1] https://www.biography.com/business-figure/bill-gates
[2] https://legacy.npr.org/news/graphics/2008/june/bill_gates/gates_timeline_04.html
[3] https://en.wikipedia.org/wiki/Traf-O-Data
[4] https://www.cnbc.com/2018/05/24/bill-gates-got-what-he-needed-to-start-microsoft-in-high-school.html
[5] https://www.washington.edu/uwit/about-us/publications/history/
[6] https://en.wikipedia.org/wiki/Dartmouth_BASIC