Actually, it's harder to understand how yosef was already seventeen when he was sold:
After Yosef was sold, we are told (B'reishis 38) that Yehuda proceeded to get married, and have three sons,from separate pregnancies (as is evident in Pessukim 3-5, where the word ותהר is used before the birth of each kid). Since pregnancy takes 6-9 months, at least 18 months have passed by the time Shela is born (and more likely closer to 27. This is all assuming there was no break between each kid being born).
We further know that Er and Onan both married Tamar, and at least Onan died because he refused to impregnate her (Pessukim 6-10), and Tamar waited until Shela was old enough for marriage before tricking Yehuda (Passuk 14). This presumably means when Shela was 13 years old, but I'll be as conservative as possible and follow the Gemara's opinion that a man can have a kid by age nine (for example Berachot 24a, and Rashi there. But this is also seen many times throughout Shas), which brings our total to at least 10.5 years.
Tamar then tricks Yehuda, and gets pregnant, and gives birth to twins: Peretz and Zerach (Pessukim 14-30). This is another 6 months at the very least, bringing us to 11 years after Yosef's sale.
Later, (46:12) we are told that Peretz had two sons by the time Bnei Yisrael went to Egypt. Even assuming they were twins, and were born after six months, and Peretz got married and impregnated his wife at 9 years old, and they were born just as they left for Egypt, that still adds 9.5 years, bringing our total to 20.5 years.
So, by this math, Yosef had to be at least 37.5 years old by the time Bnei Yisrael joined him in Egypt (even assuming he was 17 at the time he was sold).
Coming from the other side, Yosef was 30 when he became second-in-command (41:46), after which followed 7 years of plenty, and two years of famine before Yosef revealed himself to his brothers (45:11) and sends them to get Yaakov. Even if it takes Yaakov a year from then to get to Egypt, Yosef was at most 40 years old when Bnei Yisrael joined him (more likely 39, if we only count 2 years, as the Passuk implies, and as the midrash states).
So in essence, our most generous math only gives us a 2.5 year (and seemingly only a 1.5 year) gap during which we could claim the story with the dreams happened, before Yosef was sold. Therefore I would say it's unlikely, and almost absurd to claim that Yosef was over 17 when the dreams happened (especially since the 'extra' 1.5-2.5 years are needed to alleviate some of the most problematic liberties I took in shortening the years of Yehuda's family tree, as noted above and below), and it's extremely unlikely that any commentary would do so.
As mentioned above, this all works only if we assume that all marriages were at 9, not minimum thirteen [which would add at least 8 years - Shela's theoretical marriage age and Peretz's], and that all babies were born without breaks, starting at the marriage, and pregnancies lasted 6 months [if they were 9 months each, that would add over a year, for 5 births; if they weren't consecutive, that could add years], and that Peretz's kids were twins [otherwise we add at least half a year].
