Ben wrote regarding the putative age of Symeon bar Clopas in Hegesippus:
The trouble I have with believing this particular number is that it is exactly how long Moses is said to have lived. I believe Rabbi Akiva, Hillel the Elder, and Yochanan ben Zakkai were all said to have lived 120 years, as well. I think this is an honorary device, not an accurate record of a great man's lifespan.
Well, of course. I agree. As I put it on another thread, "there is nothing unusual about that [age], since this is the age that Moses is said to have died so it is said of other important (and more or less contemporary) figures in Judaism as well, i.e., Hillel, Rabban ben Zakkai (the founder of post-70 CE Rabbinic Judaism) and Rabbi Akiva ..."
I am nevertheless comfortable with the idea that all of these people lived to a very old age, whatever the actual number may have been, and in Symeon's case, he is said to have lived up to Trajan's time, and that could be anywhere from 98 CE to 117 CE. So as far as guessing his actual age goes, it's really a matter of guessing when he was born.
Akiva, for example, is thought to have been born c. 50 CE and died c. 135 CE, making him approximately 85 years old according to non-hagiographic reckoning.
By similar reckoning, if Symeon was born in 10 CE he would have been eighty-eight in 98 CE. If the idea that Akiva lived to 120 doesn't impinge on the idea that he was nevertheless a very old man, I don't see why the same wouldn't apply to Symeon. In both cases we know approximately when they died (sometime during the Bar Kokhba war for Akiva and sometime during Trajan's time for Symeon) and the date of their birth can only be guessed. And Akiva's case shows that there's nothing implausible about someone living to an old age and being martyred as a revered messianic leader in this time period.
And let's approach it from another angle by way of comparison. Josephus says that Essenes lived to be over a hundred in part "by means of the simplicity of their diet," and some Jewish Christians were similarly particular about their diet. James, for example, is said to have "drank no wine nor strong drink, nor did he eat flesh" in Hegesippus, Peter and others are presented as being vegetarians in the later Clementine writings, and Paul speaks of believers who "eat only vegetables" in Rom. 14:1-3:
Accept the one whose faith is weak, without quarreling over disputable matters. One person’s faith allows them to eat anything, but another, whose faith is weak, eats only vegetables. The one who eats everything must not treat with contempt the one who does not, and the one who does not eat everything must not judge the one who does, for God has accepted them.
So it's not out of the realm of possibility that Symeon had a simple diet that contributed to his longevity as well.
Tell me all that you know and I'll show you snow and rain.