There are basically two thoughts in the Christian realm as to the age of the earth. The millennialist view holds that the day is 24 hours as based on the statement "and the morning and the evening was the first day." However, based on that, a day would have only been 14- 16 hours. Also note that the 24 hour time scale was developed thousands of years after Genesis was written.
The second thought, "The day age theory" supports the idea that the earth is millions of years old. Scriptural support for this are; psalms 90:4 For a thousand years in thy sight are but as yesterday when it is past, and 2 peter 3:8 But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day. Always remember: when debating the creation of the universe, keep the discussion relegated to either biblical or scientific. Mixing the two will automatically loose many readers, as they are adamant about their beliefs and will not acknowledge arguments from an apposing view.