For a more mature sounding voice, I'd recommend using a more chesty tone. Try mimicking someone with a deeper voice, like say, Kohaku Merry (?) or an older female opera singer. Play around with the parts of your voice that'll produce a more mature sound (I can't explain this part verbally, you really are the only one who can find out for yourself)
Do you ever ironically imitate an older woman? Like, in an exaggerated way? That's a good start.