Just about every shaft line that offers an R flex will offer a senior/A-flex shaft. Across the 30 or so available in the Titleist catalog, the weight, launch pattern, and kick point will vary.from model to model. The only possibly accurate criteria is if your driver club head speed is 85 mph or slower, it may be time to look at S versus R flex shafts.
We can only give you an opinion whether a 40-50-55 g shaft will be best for you.

First off, who advised you? If it wasn't a qualified club fitter, it's like throwing darts with a blindfold on.
Every shaft model is unique, and the flex and weight are not based on one criteria. It's a process that truly takes some time and knowledge to find what works best for your swing. Assuming you're playing Titleist, try to find a qualified Titleist fitter with a full selection of shafts. You may have to pay a bit for the fitting but after going through it you'll be happier than just asking for a senior shaft.