This question is a bit unclear, but I'll try to answer as best I can. The question seems to ask about how age affects your ability to be hired as a professor.
TL;DR: While biological age is strongly correlated with factors that determine hireability as a professor, it is not itself a major factor.
For hiring faculty, the key question being asked is "what is this candidate's career trajectory?" and part of that is whether the achievements match up with the candidate's academic age. I believe it is illegal to consider the candidate's biological age in the US.
The key determinant of academic age is the PhD graduation date. A 25-year-old fresh PhD graduate has the same academic age as a 35-year-old fresh PhD graduate and both would be evaluated with more or less the same criteria. Obviously there will be some limit -- if your CV shows a 25-year gap between bachelors and PhD or that it took twice as long as usual to finish your PhD, people will wonder why.
The "red flags" will be things that have happened between the present and when you graduated. If there is a gap, that looks bad -- but could be explained, e.g., medical or parental leave. If you've spent more than 4 years as a postdoc, that looks bad (for computer science, in other fields long postdocs are normal).
Now if you already have a faculty or faculty-equivalent position, then things look different. You can be competitive for positions even 40 years after your PhD. However, after more than about 10 years from your PhD, you will need to apply for positions at the tenured level, rather than the assistant professor level. This means there is a higher bar in terms of a consistent research record and you will also be judged based on your record of teaching, advising, and funding.
breaking into academia as a professor in the US
? It depends where you are coming from. An established professor in, say, Canada could move to the US at essentially any age.