my DD1 stopped when she turned 5. we discussed it a lot around 4yo, and then between 4 and 5 but she really wasn't ready for it. her teeth were awful but within 6 months of stopping they all came back to normal.
she also had her teddy bear that she would hold when sucking her thumb, the teddy bear stayed and she still sleeps with it today but she stopped sucking the thumb on her own. she needed some help, lying with her to fall asleep for a few nights, and then asked me to put on bad-tasting nail polish to help her keep it up, and she got a reward after a few nights, that she got to choose herself so she was quite excited about that!