How much density is needed for fine hair to look thick & dense (not transparent) at the hairline? Have you ever seen cases with super fine hair, that achieved good density?
Normal density in a fine-haired individual (like what you had at 15 years of age) was never an Elvis hairline, and you know it. It is impossible to put enough very fine hair to get an Elvis hairline, but you can get the look of what you had when you were 15.