The majority of freshmen are recent high school graduates, so 18 would be the most common age. You would also find a substantial minority of 21 or 22 year olds, who would be veterans going to college on the GI Bill. Today, you will also find quite a smattering of middle age college freshmen, and even some seniors.
To show how much things have changed regarding college students, I went to college in the early 1970s. One of my fellow students was Mrs. A., who for those days was quite a unique person. Mrs. A. amazed everyone by being a freshmen at the ripe old age of 26. She had married right out of high school, then worked full time while her husband went to college and law school. With him then employed as an attorney, Mrs. A. started college.
In addition to just going to classes, Mrs. A. was also on the tennis team and a cheerleader. The local newspaper even did a big human interest story about her. Decades ahead of her time, nowadays Mrs. A.'s educational circumstances would not be considered uncommon.