C# Update a List from Another List

Answer 1

As I consider, you want to update only an age. Also you don't need to use Where().First() you can use just First().

foreach (var x in ListB)
{
    var itemToChange = ListA.First(d => d.Name == x.Name).Age = x.Age;
}

If you are not sure, that item exists in ListA you should use FirstOrDefault() and if statement to check it.

foreach (var x in ListB)
{
    var itemToChange = ListA.FirstOrDefault(d => d.Name == x.Name);
    if (itemToChange != null)
         itemToChange.Age = x.Age;
}