I realize that being 71 on election day 2020 means there's a good chance that she wouldn't run, but the separate question is whether she would lead the very early polls (which would begin as early as December 2016, if not earlier) for the Democratic nomination, and be regarded as the early frontrunner. Plenty of voters are ignorant about how old she is, so I don't see why she wouldn't still be strong in the polls for some time.
Her age isn't as significant as the perception she's not interested in running.
If she indicated that she might want to run for President in '20, she'd probably poll pretty well in December '16.
Even if she had softened to the idea of running for President, she might not want to express that opinion publicly so early. I doubt it would hurt her chances for reelection, but it makes her a bigger target for Republicans and fellow Democrats.
