Is it immature to say "I love you" too much to your girlfriend?

Question in the title really. My girlfriend and I are both 21 and going into our final year at university. We'd exchange the magic three words (both of us do it) anything from 3 to about 15 times a day. I have a feeling it's something people do when they're in high school but then just grow out of. From my friends I'm under the impression it's something you say less and less as you grow older because of maturity (and less insecurity, etc) and so it becomes even more special when it is said. Is there anyone here who's 21+ in a similar relationship that says it all the time to their boy/girlfriend?

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