gddunton wrote:

A sequence is a series of terms t1, t2, t3 up to tn. The sequence starts with t1, hence you start with n=1.

yes we can say a sequence is series of terms t1,t2 but that doesnt mean we start with n = 1. what if we take -ve integers to check?

lets consider this sequence S ={ -6,-4,-2,-1} here terms t1,t2 will correspond to -ve numbers

however found this discussion in the below link. Hope this helps

http://gmatclub.com/forum/m14-q3-why-isn-t-0-a-term-83934.html