How to fill null values with mean

I have data:

print (df)
      Sex  Age  SbSp  Parch
0    male   22     1      0
1  female   38     1      0
2  female  NAN     0      0

There is some NAN value. I want to fill up with mean value.

I did use

df1 = df["Age"].fillna(value=df["Age"].mean()

But it did not affect my data set.

What is problem?