Q. Individual in his advanced age and was never married. At what point should he start to be m’kayem Mitzvah of Tzitzis with Tallis Godol, being that (due to his age) it is not likely that he will get married?

A. See question 1194, in regards to an Ashkenazi bochur, who does not wear taalis, and davens in a Sepharadi shul, where they do. We wrote that he should follow the minhag hamakom, (the custom of the place). See Or Letzion (2: 2) in regards to a Sefaradi bochur learning in a Ashkenazi yeshiva, he rules that he should wear his tzitzis on the outside, as everyone else does. See also a similar question in Toras Hayeshiva (p. 18).
Horav Shlomo Miller’s Shlit’a opinion is indeed to follow the minhag hamakom. However, he added, that if there is already a distinct and defined group of people not wearing a taalis, he may join them.
See also question 1017 in regards to the validity of the mitzvos of single people; we wrote that the mitzvos definitely count. There are very few mitzvos and traditions that a single individual after bar mitzva does not have to observe and fulfill. Some of them are;
1) Wearing a Talis Godol when praying in many Ashkenasi congregations or according to some opinions,
2) In some communities, not performing birkas cohanim (Shibolei Haleket 23 ) since an unmarried person is not considered as being truly content (Yevomos 62b).
3) Being the choicest individual for leading services and blowing shofar on Yamim Noraim, 4). If he is a cohen not sacrificing a korban in the Beis Hamikdash (Zohar – Vayikra p. 5, Naso 145,) and a few more. However, definitely the importance, quality and excellence of the Torah learned and mitzvos performed when done with kedusha and holiness, is boundless.
Horav Shlomo Miller’s Shlit”’a opinion is that if one davens and belongs to a shul where both traditions are equally accepted, he should preferably wear a taalis.
It is difficult to determine a fixed age for wearing a taalis, when davening in a supporting shul. The Rov suggested that once the issue has become meaningful enough that one has to consult a Rabbi, it is likely the right time. (Some communities have singles wear a taalis, together with a hat).

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a