Hi Tim,
Welcome to the forum. Don't worry about asking lots of questions; the forum is the place for that.
I only do short break European coach trips so my answers will be based on those experiences.
Firstly, and this is often a surprise to those new to JY, but it maybe that not everyone on the trip will be single. Most on the group will be, but there may also be a few marrieds travelling alone. The marrieds (including myself) usually go on these breaks for a variety of personal reasons. In my case it's simply that I have a long haul holiday with my wife and then we like to do our own thing for short breaks. Occasionally amongst the group there may also be a couple of friends or mother and daughter etc who go on the trip together but as single travellers. However the vast majority of the group will be indeed be single.
The typical age range is usually 40 plus, most will be in the 50 to 70 years age range but there will also be those who are younger and older. At 45 I doubt you would be the youngest on any trip but equally you are unlikely to be the oldest.
The trips I've done have had 30 to 40 in the group and there has always been more ladies than men.
I haven't done a Discovery Weekend but they do seem to be very popular and informative. I know that some people have done several Discovery Weekends as they enjoyed them so much. It does seem to be a sensible way of getting a feel for JY breaks if you are at all uncertain.
I personally would recommend doing a Discovery Weekend or one of the short breaks to see if it's for you and then take it from there.
I will say the JY trips I've done have been well organised and great fun. I've met some really interesting people on these trips and would thoroughly recommend them.
If you have any more questions just ask - I'm sure one of the forum members will be able to give an answer.
Hope that helps.
Julian