'Well into his 35s' doesn't make much sense. If you say that someone is well into their thirties, however, it means that they are in their middle or even late thirties.

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.

Maybe tend to?Otherwise is ready to?