"Very old wizards get a bonus to hexing simply
from their age, which is why Harry can routinely
drive a Volkswagon Beetle while some members
of the Senior Council still have their sanctums
decked out like it’s 1599. This chart assumes
that the wizard is around fifty years old or
younger—as a guideline, set the “1 Power” category
wherever the wizard would start finding
the technology truly alien (so that special wizard
who was actually born in 1599 could probably
hex everything on the chart at 1 power)."
This assumes intentional hexing; a 300-year old character would probably start being mystified at Industrial Age stuff. So around level 7 on the chart would probably be your baseline for deliberate hexing. (Catapults are much older technology than anything on the chart, but such a character could probably still affect them with a sufficiently powerful hex, and 15 shifts relative to level 7 would likely affect anything even remotely technological no matter how ancient, so long as moving parts were involved. Levers might qualify...) Deliberate hexing is a spell, with the required shifts determined relative to your position on the chart.
Accidental hexing is a compel:
"Accidental hexing is handled as a function of
compels, usually of the wizard’s high concept
(or any appropriately emotional aspect)—something
gets hexed, it puts the wizard in a bind,
and that means the wizard’s player gets some
fate points."
So when the wizard becomes emotional or starts flinging magic around, the GM can choose to throw this at them. But note that for it to be a compel, it has to somehow disadvantage or limit the options of the character being compelled. So having the power to the streetlights die just when you're trying to sneak into a building isn't going to occur as a compel ... unless the GM decides that in doing so it alerts the clued-in special-ops commandos with the infrared goggles to the presence of a magical threat. If so, enjoy the FATE point! On a normal basis (when the wizard is calm and not casting), however, no accidental damage should occur. Unless the GM thinks it should.