As they say, a healthy mind and a healthy body live hand in hand. It seems you have been in good health for all these years and yet you are recently wondering how health declines with age. As with any thing in life, everything has a peak and a fall, thus our bodies also go through a cycle of growth. Things will change over a period of time and depending on how you use it and keep it, it will respond. As you say, you have been exercising for the last 18 years, so you seem to be OK. And as with anything, the best way to go about change is to accept it as it comes.
Keep up with the good health and positive attitude and you will flourish in good stride. Do not worry too much about the numbers you seem to be doing alright. You could go for an all round check up to get an indication of how you will progress.