Honestly, it all depends on the person buying the bike.
Age is definitely a factor but it's not the be all and end all.
You have to profile the buyer and get a sense of his/her personality.
You can tell a lot about a person from a 5 minute conversation.
If you have a 30 year old showing up to your house claiming they have all kinds of experience and you can tell by their attitude that they are irresponsible/immature hotheads and would likely do something to endanger their life, would you feel better about the sale? I wouldn't.
And on the flip side, if you have a 25 year old show up, telling you straight up that they're new to riding and they will take it easy and take training courses, buy all the right gear, etc, would you feel better about the sale? I would.
Based on first hand experience, there is nothing wrong with starting on an SS or a big CC bike.
My first bike is my current 2006 GSX-R 750, I took it easy, learned the bike, learned my comfort level and I'm here two seasons later going into my third.
I would say go with your gut, just keep in mind that a person could hurt themselves on any bike and the outcome could be fatal.
It's just that a big CC bike will **** you up faster. lol