Approximately how old is Boo in beginning?

We're not absolutely sure how old Arthur 'Boo' Radley is at the start of the book, but we can certainly make a fairly accurate estimate, nonetheless. In Chapter 1, Miss Stephanie Crawford retells the old story about Boo stabbing his father in the leg with a pair of scissors one day when he was cutting out articles from a newspaper. Boo was 33 when he attacked his old man.
We don't know for sure exactly when the incident happened, but Scout tells us that it took place "barely within Jem's memory." We know for a fact that Jem's nine years old at the start of the book, and being as how someone that age wouldn't have more than a few years' worth of memories, we can reasonably infer that Boo is in his late thirties.