The first window of infectivity as described by Caufield is between:

Initial acquisition of mutans streptococci by infants: Evidence for a discrete window of  infectivity. Caufield et al described a discrete “window  of infectivity” during which infants acquired mutans streptococci (MS) from their maternal host.  This “window” opened at 19 months and extended to 31 months, with a mean of 26 months. During this period, the prevalence of MS was seen to rise from 0% to 82%. Caufield hypothesized that the discrete nature of initial MS acquisition was directly related to the presence of nondesquamated hard surfaces, namely newly erupted teeth. Empirically, if the acquisition of these cariogenic bacteria could be blocked during this period, an individual may never acquire these organisms and thereby experience lifelong immunity from dental.