Yes and no .I'm not saying that having alighter bike is a waste ,it's not, but it's just one side of the problem.As we age we loose muscle and strength ,the bike looses nothing. Exercise with weight done correctly replaces that muscle and strength the body needs for lifting.It also has many other benefits when it comes to the quality of life in our "Golden Years"The only downside is most folks are not interested in taking the effort.
Improving one's physical condition is not going to make it a cake walk up the stairs but it will make it much,much easier.Also, it matter's not how old,what gender or how much we carry around our middles, exercise and getting in better shape is the right tonic for "getting on".