Shame on that first LBS.
You weigh less than most of the men they probably fit on road bikes. And a smaller frame is automatically more rigid. You're not even close to being "too big," if there even is such a thing.
And, if you're properly fit to your bike, you won't be "hunched over." In fact, it's pretty rare for a woman to be "hunched over" on a road bike, because our upper bodies tend to be so much shorter in proportion to our lower bodies than men's. It's much more common that a woman will be excessively stretched out.
If you don't have good core strength - which is the only thing that your age might possibly have to do with anything at all - you can improve that. As we get older, everyone (male or female) loses muscle mass if we don't do strength training, and the first things to go are the abdominal wall and the glutes. So if you're not doing them already, lots of planks and bird-dogs and variations of those will help your stability - not only on the bike, but in everyday life.
Other than that, being 49 has nothin' to do with nothin'. Except if you're lucky like me, you don't have to worry about bloodstains on your shorts any more. 
PS - if you want wider tires than 23s, think about 25s, or possibly 28s. 30s and 32s are (1) very, very wide and heavy and (2) almost certain not to fit on a road frame. Also, if you're thinking about putting different sized tires on, the wider one should go in the back, not the front.
Speed comes from what you put behind you. - Judi Ketteler