The youngest reference I have come across is in the 1992 Watchtower (March 1st page 27) - “In the summer of 1946, I was baptized at the international convention in Cleveland, Ohio. Although I was only six years of age…”
Interestingly this experience from w92 3/1 pp. 26-30 Pursuing a Goal Set at Six Years of Age - https://wol.jw.org/en/wol/d/r1/lp-e/1992168 is also the very last place in the Watchtower mentioning the BETH-SARIM
Often we preached near Beth-Sarim, which was where the ailing president of the Watch Tower Society, Brother Rutherford, had spent the winters before his death in 1942. We visited regularly and had dinner with the full-time servants there. Such happy visits made me decide that this was truly the kind of life I wanted. I then made Gilead School and missionary service my goal in life.
https://www.jehovahs-witness.com/search/?q=beth-sarim
Oh-my-God!