Human language acquisition begins before age 2. Most 2 year olds have an understood vocabulary of at least several dozen words, and a spoken vocabulary of typically a dozen words.
Further, by 12 months, sound set discrimination acquisition is near-complete. (It can be retrained later by use of neurolinguistic programming techniques, but such techniques are best done with computer delivery....)
So, being a 2 year old at arrival (between 24 and 36 months of age), if his mother hadn't already taught him Elvish, then he'd never be truly a native speaker of Elvish, though could become quite fluent in it.
The books seem to imply he speaks Westron natively, but never explicitly state whether he learned Quenya and Sindarin prior to moving to Rivendell, and the safest presumption is that the Elven languages are not native to him. It's clear as well that he is fluent in Westron, which means he had to fairly continuously find use for it in addition to Sindarin and Quenya.
Note that adoption at age 2 does develop near-native fluency, but still typically results in some minor phonemic deficiencies. Since we know Quenya and Sindarin share almost the same phonemic sets, it matters little which he learned first of these two; we don't know Westron in the same way, since the Professor never created Westron as a Conlang. We can not, therefore, determine if he'd have a major, minor, or no impediment from lack of phonemic awareness.