Happy Hour Attendance by Gender and Age: 25-34 Most Likely, Over 55 Least Likely

An equal number of men and women reported attending happy hours with co-workers, with workers aged 25 to 34 being the most likely to attend and those over 55 being the least likely to attend.

Here, 'likely' refers to 'possibility' or 'tendency.' Therefore, in this sentence, 'most likely' indicates the highest probability of attending, and 'least likely' indicates the lowest probability of attending.