It is noted in the ME wiki that Salarians mature much faster than humans.
Given that they live typically to an age of 40 years they would likely mature a lot earlier than a human, but how much less?
At what age is a Salarian considered an adult?
I don't think they ever say exactly when a salarian is considered an adult, but we can do some fuzzy math to get a rough idea:
According to the Mass Effect Wiki, Mordin Solus was born sometime in the 2150's, and was recruited by Commander Shepard in 2185. That means that he was between 25-35 years old when he was recruited. We'll take the average and say he was born in 2155, so he's 30 "now." Before that time (working backwards) he was a doctor in Omega, a member of the Genophage renewal STG team, a solider/commando of some kind before qualifying for STG (giving rise to the amazing quote "Have killed many, Shepard. Many methods. Gunfire, knives, drugs, tech attacks, once with farming equipment."), and then he must also have been a student for a while before becoming such a respected scientist.
Mordin's operation in Omega seems well-established, but given that only certain people seem to have heard of him, we can assume it's fairly recent. Call it 6 months. The way Mordin talks about his fond memories of the Genophage project, it sounds like a more substantial amount of time, probably at least a year, maybe 2-3. His military career (that quote again) seems to have gone on for long enough that he's seen plenty of combat despite living in a generally peaceful era, so call it 4-5 years of service. He's obviously a genius even among salarians, so his education may have been more rapid than normal (the salarian Doogie Howser), but even he had to have gone through at least a few years of study to know so much in so many fields, so call it 3 years of "graduate study" after whatever standard education salarians go through.
Now, these numbers are admittedly my personal estimations rather than hard facts from the canon, but given those numbers, Mordin started "graduate school" when he was about 18, equivalent to a Human of 23-24. Thus, an 18-year-old Human is probably equivalent to a 14 year old Salarian, maybe younger if I'm underestimating the length of any of those phases of Mordin's life.
So I'd say salarians probably "come of age" around 13-14.
But again, that's just an educated guess.
UPDATE (Oct 2014): I just noticed a line from ME2: Mordin mentions that a nephew recently got tenure at a university, and that he just turned 16. He's described as a "promising geneticist," but Mordin specifically says that he's not a genius or a prodigy. (Or rather, he is, but the timing of his getting tenure is unrelated.) Unless the Salarian academic world is radically different from our own, this likely means that Salarians come of age MUCH younger than my previous estimate.
Given the time it would take for a Salarian to go to college, probably grad school as well, then serve as a professor before achieving tenure, that means Salarians probably come of age more in the 7-9 year range.
Again, just an educated guess, but with more data from the canon comes a more reliable estimate.