My vague phrasing, sorry. I believe Tepper was probably in her mid to late fifties when she started publishing. I don't know if KING'S BLOOD FOUR was her first in the early 1980's, but it was the first of hers I'd read. It started an ambitious series with a lot of different characters, and some savagely wise observations about humanity. She hasn't backed off, either.
Age and experience can bring a depth to writing that late teen and early twenty-something writers might not have. ERAGON and TWILIGHT each succeeded for different reasons, not just the relative youth and inexperience of their writers. Age was a marketing factor, I believe. But Paolini had his parents backing his indy publishing run. They knew what they were doing. Meyers had the luck to hit the sexy-vampire field while YA readers were looking for something like Anne Rice and Charlaine Harris, but less threateningly explicit. She found a good agent who saw possibilities. To give Meyers credit, she had to learn how to write on the job, with her later novels -- something she still hasn't perfected.
Most agents aren't going to care about the age of the writer approaching them, just the quality of the writing. Age confers a responsibility, certainly. If you don't have dewy youth on your side, you'd better have a knockout story. A seventy-year-old would be expected to have something better than ERAGON or TWILIGHT.