I agree with an earlier post that there isn't no right or wrong answer to this question... there are certainly exceptions to the rule and not all 18 year olds are fun and energetic...
I pledged my sorority at the age of 18... in fact I was the second youngest in my class! I am very active... however, the oldest girl in my pledge class is 25, but she pledged when she was 23... she's very active when she can be... but work is very important to her... which shows her dedication...
There were rumors about a 42 year old woman going through recruitment at my school this year. I think she registered, but I didn't see her at any parties...
I think that there shouldn't be limitations to going through recruitment... as long as the person understands that they may or may not get a bid considering the circumstances and considerations of age, scholastic aptitude, campus/community involvement, character, ratio of Freshmen/Sophomores/Juniors/Seniors going through recruitment, etc...