You can't go by what a single agent says. Interview at least three agents, all of which should work for different Realtors. Considering how uncomfortable you are with the first agent, you may want to discard his analysis altogether.

Find a different agent! This guy is telling you in his own way he doesn't want to do the leg work needed to find proper comps which in my experience means he won't do a good job marketing your home.

Call other realtors but do not let them know your expectations, because often, they will tell you want you want to hear. Tell them all you want a very honest price. Many realtors "buy" listings by telling sellers that their homes are worth more than they are in order to get the listing. Listen carefully, use your instincts, and ask for references.

Pay for an appraisal. Do a web search - you will find some appraisers in your area. It usually costs somewhere around $300 or so. Tell them you are selling and want to know a price to sell at. Be there when the appraiser comes by - so you could pick their brain about what is selling while they are measuring rooms and such. They do appraisals on homes just before closing, so they are in tune with the recent market activity in your area.

I did this when selling and it really helped me set a realistic price.

Creeksides advice should also be taken, and do the appraisal as an option if you still don't feel good about the other agents' advice.

You don't want to price too high or too low. My parents had an agent that suggest they price too low by about 5-10% and the comps they gave were not really in the same area and were much different in style, age, type, etc. My parents ended up pricing higher than the agent suggested and even SOLD for higher than the agent's suggested asking price and sold within the average timeframe for that area.

Some agents want you to price it a bit lowish because they know this will move the home. They might not even realize that they are on the low end of the suggested price range. Many of them have seen homes priced too high that sit so they want you to price it so it moves. Can't blame them in some ways, but you don't want to give away ten grand by pricing too low if you don't have to.

But you have to be careful not to price too high either - then you have a home for sale for way too long with no offers or lowball offers. Then eventually you might sell for lower than what you could have originally sold for because the home had been on the market for so long.

Correct pricing is very, very, very important.

As long as your home has been upgraded, updated and maintained, the age difference does not matter. An appraiser would use the comps closer to the subject house, even though there is an age difference. The lenders will expect this too.
Basement valuation is different based on where you live, and the type of basement. If it is a daylight walk out, here, in NC, basements are valued the same as long as the finishes are of the same quality as the above ground floors. If they are below grade on all sides and finished in like quality as rest of house, they will fetch about 2/3 of the value of the above ground floors. If they are totally unfinished, they fetch about 1/3 of the value.
Your pexiglass porch would not be counted as additional heated living area, because to do so, the area has to be finished off with typical, conventional materials. Plexiglass is not conventional... insulated, glass windows are.
Just get a couple of more opinions.