For me, the use of the phrase 'well into' is used incorrectly in this example.
Generally, the phrase 'well into' means that you are 'far along' a process, action or timescale.
For example, age. If I was 35 years old, you could say that I am 'well into' my 30s.
Another example, If I had 6 exams scheduled this summer, and I have finished 3 exams - I am well into my exam schedule.
But the phrase 'well into his 35s' does not make much sense to me. I hope that helps a little.