Breadcrumb: How old?

Num. 8:24-26 says that Levites aged 25-50 would serve in the tabernacle. Num. 3, however, said that it was only men 30-50 who served. What was the real starting age, 25 or 30? What did they do for those five years from 25-30? Rashi theorized that there was a five-year apprenticeship period before the Levite men could be fully-trained tabernacle attendants. It makes sense that there was a training period required before the Levites could be trusted with the holy equipment, especially if the alternative is saying that the Bible is wrong. Who would say something like that?