Well, one thing about Peace Corps - unless you have a good (i.e., medical) reason for hot vs. cold, I can pretty much assure you that they probably won't really consider that.
I expressed a strong desire to go to a place that wasn't too warm, and ended up being invited to Ghana....
That being said, it was a great experience, and I can't imagine having gone anywhere else.
As for age - I wouldn't worry too much about that as most countries have a wide range of ages in the training groups. In Ghana, ages of volunteers varied from 19 to 63, with pretty much everything in between.
I'm seeing a similar trend this time through as well (in terms of ages).