Began is the past tense of begin, while had begun is the past perfect tense. The past tense indicates that something happened, while the past perfect indicates that the thing happened and was completed. It's admittedly a bit odd to talk about the beginning of something being completed, but you can understand it to mean that the actual beginning was sometime even further in the past. So, if you say:
Walker Lee began his career in hand forged ironwork at the age of 30.
then you're talking about the time when his career actually started. The reader will understand that Lee was 30 when he showed up at the forge for his first day of work, and the expectation will be that you're going to continue talking about that time in Lee's story. But if you say:
Walker Lee had begun his career in hand forged ironwork...
then you're talking about a point in time after the career started, and the only reason to talk about that moment is to introduce some new information. For example, you might say:
Walker Lee had begun his career in hand forged ironwork when, at the age of 30, a chance encounter led him to a remarkable innovation.
Here the reader will understand that the career was already started by the time the encounter occurred.