Advocates of "family-integrated worship" -- a fancy term for keeping kids of every age in church services rather than shuffling them off to the nursery/crèche, Sunday School (UK), or Children's Church (US) -- generally claim their practice as the historical one up until rather recent times. "What we advocate," writes one proponent of family-integrated worship, "is nothing new, but is rather the practice of historic Christianity. [...] It was not until the philosophy of age-segregated education inflitrated [sic] the educational regimen of the nations, and then was adopted in the churches, that the people of God had to face so many family disintegrating forces." With considerably more levity, the brilliant forces behind Lutheran Satire recently named age-segregated worship as a modern invention (courtesy of Mr. Thompson and the Vicar) fundamentally at odds with "the multi-generational model of worship so foolishly employed by all the Christians in the history of forever until five seconds ago."
Whatever the merits of including children from early on in church services (and there are, I think, many), I'm not convinced the evidence for such being the unequivocal practice of previous ages is all that strong. Too often the argument from history on this matter seems to be one from silence more than anything else. Moreover, Kirk session records from the sixteenth-century suggest that Scottish church leaders at least thought keeping younger kids out of church services might be in the best interest of the whole congregation, even if there is, admittedly, no evidence that they spent any effort devising a wholesome alternative to corporate worship for the youngsters.
In her fascinating work The Culture of Protestantism in Early Modern Scotland, Margo Todd observes that infants and young children were "systematically excluded... from Sunday sermons" in sixteenth-century Scotland in the interest of making sure that more mature parishioners were hearing and benefiting from the preaching of God's Word. Indeed, presbyteries and sessions went so far as to impose monetary fines on parents who breached ecclesiastical regulations against bringing potentially distracting youngsters to Sunday worship. Glasgow's churches apparently made 8 years of age the "cut-off" for attending the sermon. Aberdeen excluded children from corporate worship until they had reached school-age and demonstrated their ability to "take themselves to a seat." In Kingsbarns, just south of St. Andrews, the laity were ordered not only to exclude "little ones and young children" from worship, but to keep them enclosed at home (to prevent them from distracting worshippers by "running up and down" and making a racket in the vicinity of the church building). Church legislation in Perth in 1582 threatened parents with a hefty fine of six shillings and eight pence and/or imprisonment for bringing their "bairns... [to] kirk in time of preaching."
As Todd astutely points out, the exclusion of infants and children from worship presented a difficulty when it came to baptismal services "since one could hardly exclude the baby" to be baptized from such. Perth legislation of 1587 offered a resolution to this issue by ordering that infants "be holden in some secret place til the preaching is ended" and then brought forward for baptism, lest the crying of the baptismal candidate create "din in time of preaching, so that others incoming thereto are stopped from hearing."
Such efforts by early modern Kirk authorities to regulate the attendance of infants and young children at church might prove to be a historical anomaly. But I suspect that some digging would demonstrate that early modern Protestant churches elsewhere engaged in similar exercises. Of course, we need not necessarily follow the lead of our (Scottish) forebears on this particular issue. Personally (for what it's worth), I'm in favor of including kids in worship from pretty early on. I suspect that carpeted floors and the amplification of the preacher's voice has made some (though not all) of early modern kirk sessions' worries about the distraction children might cause other congregants less pressing in our day. Regardless, it seems to me that such worries shouldn't be permitted to trump the reality that faith comes through hearing, and so the benefit of situating our covenant children under the authoritative preaching of the Word of Christ from their earliest days (Rom. 10.17).
Perhaps the most appropriate lesson to be learned on this point, then, is simply not to make assumptions too quickly about how Christians did things in the past. It's all too easy to project our own ideas and customs on to persons or groups that inhabit days gone by, and then to turn around and claim historical precedent for our ideas and customs on that (illegitimate) basis. We must, rather, engage persons from the past truthfully and charitably. After all, the Scriptures that urge honesty (Ex. 20.16) and charity (1 Cor. 13.7) upon us in our interactions with others contain no qualifications about whether the "others" in question are living or dead.