"SUNHKAN aor. ind. act. SUNIHMI (# 5317) to bring together, to understand, to comprehend" (Rogers and Rogers New Linguistic and Exegetical Key, p. 114).
Moulton-Milligan say that the "literal meaning" of SUNIHMI is "bring together"; metaphorically speaking, it conveys the sense "perceive" or "understand" (Vocabulary of the Greek Testament, p. 607-608).
Finally, the NIDNTT confirms this way of approaching Luke 2:50:
CL & OT 1. In cl. Gk. the vb. syniēmi originally meant to bring together, a meaning not found in the NT. Fig., syniēmi means to perceive, take notice of, understand, comprehend. The noun synesis originally meant a joining (e.g., of rivers); then, in a transferred sense, the faculty of judgment, apprehension, understanding, insight, comprehension. Neither vb. nor noun acquired any great philosophical importance. The adj. synetos means quick at apprehending, clever; also intelligible. The opposite is asynetos, stupid or unintelligible.
In [Luke] 2:47 the insight of Jesus at twelve years of age is the subject of amazement, and there is no doubt that such insight is regarded as a gift from God. Conversely, his parents' failure to understand (2:50) must be seen as the opposite. In addition, it is the risen Christ who enables the downcast disciples on the Emmaus road to understand the Scriptures and to grasp the fact that his sufferings were foreordained by God (24:45).
This brief exercise illustrates how when we're faced with two or more possible options for translating a Greek word, the deciding factor will be context and passages in which a word is similarly used by other writers. We must also consider the synchronic features of a word--not just the diachronic features.