I've recently encountered the term "lolicon" being used to refer to a loli enthusiast. This contradicts what I thought I knew for many years. I thought "loli" refers to the girl in the drawing while "lolicon" refers to the kind of art that contains lolis, e.g. "I'm a fan of lolicon because it contains lolis." Is the correct sentence actually, "I'm a lolicon because I enjoy lolis."? Are they both correct where "lolicon" means one thing while "a lolicon" means the other?

Both are correct; it depends on the context.

"lolicon" is a shortening of the term "Lolita complex", which was originally a term for attraction to young or prepubescent girls. When it hit Japanese, it was shortened to "lolicon" as generally tends to happen with long terms. In Japanese, nouns and adjectives often behave quite similarly, so the term quickly came to be used both as a noun for people who have it, and an adjective for things pertaining to this attaction. Used as an adjective, we get terms like "lolicon manga", "lolicon art", etc. Since when taken in context, terms like "manga" and "art" are often redundant or unnecessary, they're usually dropped. In Japanese, the term "lolicon" is also used for the attaction its self as well, but since "Lolita complex" is already an English term, we generally tend to use that term instead for the actual condition.

Similarly, "loli" is just a shortening of "Lolita", the name of the titular character in the novel Lolita by Vladimir Nabokov where Lolita complex gets its name.

Thanks. This explains everything actually.

I was just wondering about this. Aside from lolicon, are there any other age group categories?

baby - Too young to walk or talk, wears diapers.
toddler - Younger than kindergarten, explicit images are not allowed on Gelbooru.
loli, shota, child (for rating:safe posts) - children and early adolescence.
adult, old, older, young, teenage - used for characters who are not depicted at their usual age.
milf - mature woman
old_man
age_difference
father, mother
"school_uniform -child -loli -shota" will result in mostly teenagers.