Return df with just the age column cast to integer type, leaving every other column's dtype untouched.

Input

  age  weight
0  12    41.1
1  13    34.5
2  14    83.2

dtypes:
age        object
weight    float64

Output

   age  weight
0   12    41.1
1   13    34.5
2   14    83.2

dtypes:
age         int64
weight    float64