It means that when you are a child, you are surrounded by family and friends, you don't have to think about much, and everything is provided for you. When you get older, your friends get married, your parents get old and now they need taking care of, you have to get a job and you eventually realize, you'll never become a rock star as you intended as a kid. As you get older, you wake up to reality, and you realize that life is not all about cakes and kittens (warm), and that you are just a cog in the machinery (cold) of modern society. It most definitely has nothing to do with physiological structure of the body.

As an old person myself, I can tell you that you feel cold more than when you were young. However, there is also a second meaning here - that the wolrd becomes less friendly, less of a joyful experience. I'd say it depends on the person.

God has given us not only the faculties implied in the eyes, the tongue, & the lips, but has also given us the judgment by which we can choose our way----Love God truly, God's presence is never absent--& you will become, on the contrary, warm with the Wisdom Light of God, never cold---never cold like a poisonous reptile, never cold-blooded.