Y1961 Y1962 Y1963 Y1964 Y1965 Region 0 82.567307 83.104757 83.183700 83.030338 82.831958 US 1 2.699372 2.610110 2.587919 2.
We can find the the mean of a row using the range function, i.e in your case, from the Y1961 column to the Y1965
df['mean'] = df.iloc[:, 0:4].mean(axis=1)
And if you want to select individual columns
df['mean'] = df.iloc[:, [0,1,2,3,4].mean(axis=1)