Compensating for weakness
Identify resources for seeking help and advice. And especially important, understand yourself, your strengths and weaknesses, your motivations and way of "being in the world." Own it all, the good and the bad, and manage yourself accordingly, shoring up and filling in your weaknesses with resources or other people for whom your shortcoming is a strength. And don't be intimidated by doing that. To do this, yes, it takes tons of introspection.
I have found that by the time I was a CEO I no longer consulted mentors when dealing with business decisions. Instead, I had identified certain advisors who, through personal experience and observation, possessed good business sense and specific areas of expertise that I respected. Some are peers, some are older. In return for a lunch at a downtown business club, or breakfast, or a round of golf at the country club, I'll seek advice or try ideas on these advisors.
When certain expertise is needed, I'll tap outside legal counsel, accountants, professors, technical consultants, etc. I'm not sure I can classify these advisors as "mentors;" however, an outside perspective is useful in decision-making.
Using old contacts
By knowing the business better than anyone else, I became a resource and expert both internally and externally which helped me gain credibility and allow others to see beyond my age. The biggest challenge for me was not having a peer group to bounce ideas off of on a regular basis. I had a Board that I met with every six weeks or so, but it was not like being surrounded by people of the same genre. In contrast to investment banking or package goods where they recruit lots of talented people every year, I was the only MBA there. I ended up using my friends, professors, old bosses/workmates as sounding boards when I needed advice.