2 Timothy 3:14-15 does not say that Timothy understood (from infancy) what he heard from the scriptures. The point that Paul makes is that Timothy was brought (up) hearing the holy scriptures of the old testament (the new testament not yet being written when Timothy was an infant).
εἴδω
eídō. To see. This verb is obsolete in the pres. act. for which horáō (G3708), to see with perception, is used. The tenses derived from the meaning of eídō form two families, one of which has exclusively the meaning of to see, the other that of to know. ...
(B) In the sense of to perceive, be aware of, understand. Followed by the acc. of thing (Mat 12:25; Mar 4:13; Mar 12:15; Luk 11:17). Followed by hóti (G3754), that, with the indic. (Mar 2:10; Luk 5:24; Joh 6:61; 1Jn 5:13). Followed by pṓs (G4459), how, in what way, with the indic. (1Ti 3:15). Before an indirect question (Eph 1:18). Oída, perf. tense of eídō, expresses the knowledge which comes from one's state of being, intuitive knowledge.