Well football is never only about linear progression, he could be another late developer you know.
For example, Vardy was an average player before he reached 28 too, spending most of his time in lower division, nothing special. His first season in PL, only scored 5 goals, very poor return for striker. You can draw up all kind of stats with xG analysis or whatsoever, he is still very average. By the time he reached 28, he suddenly managed to scored 28 goals out of nowhere. Purple patch? He then went on become one of top goalscorer in PL in next 6 years or so, up to this very moment.
So tell me, why do you assume there absolutely no chance for Lingard for doing something similar?