pandas group by and find first non null value for all columns

Use GroupBy.first:

df1 = df.groupby('id', as_index=False).first()
print (df1)
   id   age gender country  sales_year
0   1  20.0      M   India        2016
1   2  23.0      F   India        2016
2   3  30.0      M   India        2019
3   4  36.0    NaN   India        2019

If column sales_year is not sorted:

df2 = df.sort_values('sales_year', ascending=False).groupby('id', as_index=False).first()
print (df2)
   id   age gender country  sales_year
0   1  20.0      M   India        2016
1   2  23.0      F   India        2016
2   3  30.0      M   India        2019
3   4  36.0    NaN   India        2019