How to group data?

Dear Forum members I am a new user and I need your help.

I am a biology major and have never done a lot of programming in Excel or any other software. I have to work on a statistical data presented to me. This is a clinical trial data and is quite big to make any changes manually.

So, I have different patients, they were given different doses of a medicine and the hypothesis has to be tested that factors like age, weight or race have any effect on the effect of the drug.

I have to perform frequency distribution, histograms, comparison between various covariates like age, weight, race etc. There are mutliple dose data available for each person which is represented something like this:

A
Patient Number Concentration Age
1 10 20
1 9 20
1 8 20
1 7 20

2 10 21
2 9 21
2 8 21
2 7 21


Now if I have to know how many 20 yr old patients were there in the study, how will I know that? The list is hundred patient long with 8 dose concentration data for each patient. How can I know the right frequency distribution ? bacuse I have to plot this information as histograms.