The gatherings and the walks are different experiences. World Youth Day is a week of catechesis, a vigil, and a closing Mass with the Pope; it is aimed at people roughly 14 to 30, with no upper age limit. The Medjugorje Youth Festival is a week of prayer in a parish setting; the Holy See’s 2024 note permits pilgrimage there and recognizes its spiritual fruits without declaring the reported apparitions supernatural. The walking pilgrimages ask more of the body and less of the calendar: the last 100 km (62 miles) of the Camino takes five or six days and can be walked in any season, and the Chartres pilgrimage covers about 100 km (60 miles) over the three days of Pentecost weekend.
Age does not decide what suits you. A young adult who wants silence may prefer a few days in Assisi or Lourdes to a festival, and one who wants to serve can ask the Lourdes sanctuary about volunteering with sick pilgrims. Someone who wants company of the same age should look first to a diocesan young adult office, a campus ministry, or a movement, since those groups travel together and usually keep lodging simple.
Before joining any group, ask who it is planned for: the age range, how much walking there is each day, and whether a priest is traveling with it. A standard parish pilgrimage mixes ages, and many young adults find that to be one of its gifts.