Although she had signed a pledge of abstinence while in adolescence, Frances Willard was 35 years old before she chose to become a temperance activist.
what's wrong with the "while in adolescence" in the sentence?
It is not ungrammatical, but it could be judged as unclear. I would prefer to say, "...of abstinence as an adolescent..."
that's exactly what i don't understand. Why isn't it clear?
It could be seen to tie abstinence and adolescence together, as the plege was a pledge to abstain during her adolescence, as opposed to adolescence being the time in her life when she signed the pledge.