ForEach extension on IList

Why limit it to IList, instead of IEnumerable?

public static class ListExtension
{
//    This is unnecessary as any collection that implement IList will also implement IEnumerable
//    public static void ForEach<t>(this IList<t> list, Action<t> function)
//    {
//        foreach (T item in list)
//        {
//            function(item);
//        }
//    }

    public static void ForEach<t>(this IEnumerable<t> list, Action<t> function)
    {
        foreach (T item in list)
        {
            function(item);
        }
    }
}

--- I believe that the reason it is not an IEnumerable extension is they did not want operations that could mutate data as part of the library, and there was not enough benefit to justify, it is simple enough to write your own. --- Also (in response to #2) using the extensions, makes it semantically clearer.

var studentList = new List<student>();

foreach (var st in studentList.Where(st => st.Age != 0))
{
    st.AgeInMonths = DateTime.Now.Subtract(st.DOB).TotalDays / 30)
}

or

studentList.Where(st => st.Age != 0)
           .ForEach(st =>{ st.AgeInMonths = DateTime.Now.Subtract(st.DOB).TotalDays / 30); });