There are two factors to this: Aldrin's longevity and the date of the Mars landing.

Aldrin's longevity seems to stem from his mother's side of the family, particularly his maternal grandfather's. A lot of his ancestors from that part of his family lived into their 80s, and two even lived into their 90s, and this was all during the 1700s-1800s. This is certainly a somewhat good sign. Since all these people were living during a time when it was much harder to live a long life, their true genetic potential may have been higher than what their lifespans showed. Therefore, Aldrin could feasibly live into his 100s as he has lived in a time of vaccines, advanced medicine and health research, etc. However, Aldrin is also the longest-living member of his family in its recorded history (his oldest ancestor died at 95). This brings some anxiety alongside the hope, as Aldrin is now treading truly unknown ground as far his family's longevity goes. However, as we all know, he does have a good track record when it comes to treading unknown ground, so I have faith in his ability to continue on for some more years (though living enough to see the Mars landing will definitely require a considerable amount of "some more years")

As for the date of the Mars landing, I have to admit that I'm not educated enough on the topic to make truly informed estimate, so take what I say here with a grain of salt. Google says that it is being aimed to take place in the 2030s, though I feel like it may take longer than that. I personally believe the Mars landing will occur by 2050, and certainly within my own lifetime (I'm 16, born 2009) though I could be wrong so feel free to correct me on that. Assuming the Mars landing does happen by 2040 though, which is probably overly optimistic but a possibility nonetheless, Aldrin would be less than 110 when it happens. This lines up with what I believe his upper limit is (he's almost certainly not making it anywhere past 110 imo).

So overall, Aldrin witnessing the Moon landing would require him living into his 100s (possibly feasible based on evidence but unlikely to live long enough) as well as the Mars landing taking place by 2040 (maybe even more unlikely). It would be so cool if it happened, and the chance is definitely higher than zero. But unfortunately, it probably won't happen. What do you all think?

How unlikely is it that Buzz Aldrin will live to witness the Mars landing?