In reply to Kayvon:
You're only too old if you've reached an age where you don't have anything left to learn from others! Mentorship doesn't have to be a youth learning from some wizened white haired sage - it could be someone 5 years younger than you who has been running their own business for a few years and that you bumped into at the gym. My understanding of a mentor is just someone who has something you don't and is happy to share enough of their time that you can learn from them (and in a healthy mentoring relationship, them from you too.) There are formal mentorship programmes within industry, i.e. sit down once a week and go through some paperwork on your goals and how your career is developing, however from what you've written it sounds like you're seeking something less formal?
I was listening to an episode of Rich Roll recently and one of the points made around mentorship was that your ideal mentors are probably people you already know, and it's probably different people for different parts of your life.
You focused on work/career in your post but also mentioned personal development. It would be worth splitting these things out and solidifying your reasons for seeking out a mentor, and perhaps with any other aspects of your life. This means starting from a place of introspection, looking at what you want those aspects of your life to look like in the future, what your values around that area of your life are and identifying where you are currently lacking. Depending on how easy you find that kind of self reflection, you might want help at that stage either informally or from a therapist/analyst. You can then start recognising those qualities/skills in others and identifying people who you are able to build relationships with and who would be willing to spend time with you and help you on that journey. As above, you probably already know them and they are probably going to be different people for different things you wish to work on.
For example, if discipline is holding your climbing back, then find a climbing partner who is already turning up to the crag 3 times a week rain or shine. If a lack of confidence is holding back your career, find someone who used to struggle with that but has overcome it etc.
You also haven't mentioned what it is that has recently led you to see the value in mentorship, and thinking about that might be a good place to start as well.
Also, well done for having the courage to reach out and seek help from others. It's not an easy thing to admit, particularly as one gets older.