You can quickly calculate what proportion of a particular column (data frame field) or vector meet a particular condition by using mean and an operator like so:
mean(mydata$mycolumn > 65)
The result will be a decimal, e.g. 0.21 for 21%.
To understand this it's best to start simple with a sum function:
sum(mydata$mycolumn > 65)
This goes through each item in the column and tests if it's above 65. If it is, it returns TRUE. If not, it returns FALSE. TRUE = 1 and FALSE = 0, so a sum of all those results will be a total of the number of data points which are TRUE.
Now, imagine if that sum is 21 and the total items is 100.
The mean function will divide that 21 by the total items to get 0.21, or 21%.