Well, you might have your sources but I've read differently - there are several Indian historians, specially those who belong to the materialist school of historiography - like Romila Thapar, DD Kosambi, etc - who write that this was not so. While the concept of having an age of marriage is definitely old, its only relatively recently that eighteen has been fixed as that age, with the legislations passed during British rule. Even during the Vedic period, which itself spanned a millenium, daughters used to be married 'young' - but there they had a greater degree of freedom than what they had later, when organized Brahmanical religion took over. I'm pretty sure it's sociological factors which contributed to child marriage, whenever this institution was born - and not because of 'Muslim invasions'...and like someone else has pointed out, the religion of those invaders had nothing to do with the invasions - they were Turks, or Moguls, or Afghans. They didn't rape women because of a particular religion - religion has no relevance here, in my opinion. So calling them Muslim invaders is wrong itself, in my opinion...just because certain British writers dubbed periods in our history as 'Hindu', and 'Muslim', we too fall prey to those very fallacies even today. Muslim communities had been living in India long before warriors from the Central Asian empires rode in- the Arab merchants who took up residence in the Malabar coast, etc.
Source: History course in first year of law school.