Looking into it further, I believe your issue is that you did not define the table with compact storage.  Without that, CQL3 will treat every column as a composite (as is hinted in your stack trace where you see AbstractCompositeType is the cause of the error).  Try changing your table definition as follows:

create table users (
id uuid primary key,
firstname varchar,
lastname varchar,
password varchar,
age int,
email varchar)
and compaction = {'class' : 'LeveledCompactionStrategy' }

From: Davide Anastasia <davide.anastasia@gmail.com>
Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Date: Friday, June 7, 2013 2:11 AM
To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Subject: Re: Bulk loader with Cassandra 1.2.5