Does serving the same employer, company and same line throughout one's junior career stage prepare the employee more for managerial positions?

Gaining experience is a significant step towards someone's preparation for a managerial position. Especially in case the experience is stacked by being employed by the same employer, you should be more effective as a manager in the future, since apart from the overall experience gained, you also know the management style required. Internal promotions are most common when it comes to it, since senior management requires a continuum to the managerial line.

I hope I was helpful.