Use loc:
df.loc[0:15,'A'] = 16
print (df)
A B
0 16 45
1 16 5
2 16 97
3 16 58
4 16 26
5 16 87
6 16 51
7 16 17
8 16 39
9 16 73
10 16 94
11 16 69
12 16 57
13 16 24
14 16 43
15 16 77
16 41 0
17 3 21
18 0 98
19 45 39
20 66 62
21 8 53
22 69 47
23 48 53
Solution with ix is deprecated.