The first time my now-husband told me he loved me I was totally caught like a deer in the headlights. I mean, I thought I loved him, but we really barely knew each other. It was very awkward, but I finally said "ditto." And you know what? Since he really did love me, he stuck around until I could say it with absolute certainty.
I would say it comes down to maturity, but my husband was only 20 when we had our little experience.