While I am aware that it is far more commonly phrased as "They were well into their fifties", I still from time to time run into the "in" version of the phrase. Does it really sound off to a native speaker and if it doesn't is this even grammatical?
They may be few in number, but they are out there. My non-native-speaker guess is that it may have something to do with the usage of "in" occasionally being synonymous with "into", as in "he dropped the watch in the toilet", or "dip it in sauce and then eat it". But I won't quote myself on this
I'm surprised by those examples. For me they are ungrammatical.
Here's a Google ngram showing the relative frequencies. As the graph shows, there was a peak in the frequency of "well in" around the 1960s. In the 2000s "well in" has dropped to almost nothing in comparison to "well into".