To me there are 3 components that make a person (of any age) a good social media manager: Knowledge of the tools, Business savvy and understanding that and being able to make the online world integrate seamlessly with the offiline world.
The goal of social media is to contribute to the company bottomline. It is not about social media itself (which is what a lot of social media managers think, but about generating revenue and profits for the company.
Can we please keep in mind that social media is just a set of tools in your complete business and marketing approach. Tools you can learn, business experience is something is something you get by working a number of years.
Can we also keep in mind that social media is set of software programs and can be learned like WORD, POWERPOINT or company created program. Age is not issue since everyone can learn these applications including Twitter, Facebook and others. Yes, it requires a mindset to use them properly and that gain requires business and thinking experience that is something you get with the years.
Can we also keep in mind that you use social media to reach your target audience. I understand that young people are on social media but make no mistake that older generations are there too! To reac these people TODAY you need to be able to think like them and there unfortunately the younger generation does not have those skills.
As a conclusion, I would like to say that age is not the driving factor but the ability to bring together the online and offline world to contribute to the business bottomline using the right tools for the right audience.
Looking forward to further constructive discussions