going / fifteen / cycling / been / was / since / has / he. Alex

Ответ: Alex has been cycling since he was fifteen.

1. Alex - подлежащее
2. has - вспомогательный глагол для образования времен группы Perfect
3. been - быть (3 форма)
4. cycling - езда на велосипеде
5. since - с тех пор, как
6. he - он
7. was - был
8. fifteen. - пятнадцать

Ответ: Alex has been cycling since he was fifteen.