At 63 (about 4 months before I hit 64). I stayed on for 4 or 5 years part time, teaching distance learning. Most of the time I enjoyed teaching, and it was often challenging, but it was also satisfying in lots of ways. Of course, computers and calculators were pretty taking over the field of math by that time, and the old days of proving by the traditional method was falling by the roadside also. I have 8 or 9 calculators laying around the house, but rarely use them now. I've fallen into the trap of not using it, so.....

@Hugh Manely so...you retired 4 months before turning 64? or did you finish the year of teaching and then retired.

I reached full retirement age at the time the company was offering a bonus to get rid of us older workers. I took the money and ran. I'm glad I waited because i get a lot more, both pension and SS, than I would if I'd retired early

I retired in May (birthday in late Aug.) at end of year. I found that I could still work part time a certain no. of hours and still draw SS, so my salary would be about the same as full time.

When I went to college we used slide rules and only until the year I graduated did they come out with an affordable calculator for $40. It's a TI-2510, four function made in the U.S. by Texas Instruments. I still use it as a desk calculator but replaced the batteries with an external power supply.

Retired early, at 56. Also took SS at 62. No regrets. First serious calculator was a TI SR-51A, in 1975. It cost about $100, which would be $500 today with inflation. It would do a lot of stuff, but not $500 worth of stuff.

@Nancy Hart The difficulty early on was not manipulation of typical add, subtract, multiply, divide, but rather calculating exponentials, especially fractional exponentials. My keyboard cannot show sub- and super scripts, to show examples, so I'll write it out. 2 to the 3rd power is 2 X 2 X 2 = 8. However, 2 to the 3.6 power is something else. Various schemes works, including infinite series expansions. It was slow-going, though. The later calculators did these types of work almost instantly.\ \ Frank

I recall those early TIs. I still have my HP 12C I bought in the early 80s for doing lease calculations.

I retired on July 10, 1998, at age 61, the day before my 62nd Birthday anniversary. Since then, I've used tensor calculus for resolving the Hertzsprung-Russel variables.