In the very first strip we see:
Which implies Calvin got Hobbes when he was 6. As this is the first strip, it is quite clear this is intended to be an origin story.
However, in this August 1st, 1989, strip, we see:
Which implies Calvin has had him since he was a little kid. When exactly did Calvin get Hobbes? Is there another strip or interview with Bill Watterson that clears things up?