I'm 45 years old and have very little experience on motorcycles, extremely experienced on bicycles(if it makes any   ). Some say a rookie should start out on a smaller bike in the 750 class. I prefer the larger crusiers in the 1500 to 1600   . What's the difference?  Why is a smaller bike safer? Can't you get hurt or killed just a eaisly on the smaller bike? Some say you must learn all over again anyway when you step up. Don't we face the same dangers? Why would the bigger bike be "too much bike"?  Please be specific, factual and detailed.  

I am 45 also,1963 was a great year. It is usually a good idea to get familiar with a smaller weight bike such as a 750 or 800 cc. They are sometimes 150 to 200 pounds lighter.The respond quicker to your movement and stop faster with less weight, However, If you are a big man, anything over 225 and you plan on riding two up  you may as well buy the big bike so that you don't lose money on trading up later. Just take your time and take a safety course. and never think you have mastered it. You never will. I have a 1500 Vulcan Drifter, it is very heavy and can easily get away from you. The big bikes are lathargic don't respond to you as quick as a smaller bike. Ride safe Wag  

I started at 6' 230lb, doing weights a couple times a week, and I got an 800 Drifter. Truth be to tell, it was a little big to start. Almost 700 lb. I rode it about 40 min every day for a month on back roads to get the hang of it. Not every second was pleasure, but after learning, I rode that bike from 2001 thru almost the end of 2007.

If you start on a smaller bike, the only thing you have to relearn is calibration. If you were doing it right on the small bike, it's still right. If you were using brute force to get thru, yes... you have to relearn.
And, buy the 1st one used, just in case you drop it. You don't lose as much on the bike.  

the difference is the weight of the bike as well as the overall length. The longer the bike the harder to control in tight turns in the city. The more experence the better.  The larger bikes usually ride better because of the wheel base just like a car and made more for the open road.
I am not sure that the term safety is the issue! yes, you still have the issues with other drivers just at a higher rate of speed. on the bicycles off balance is a few pounds and a foot down
may stop the fall and if not o'well. off balance with the 700 lbs pound can be a problem trying to get it off you leg or just trying to get it back up depending on your size