After years of speculation, user reports, and conflicting information, we have discovered official confirmation from Apple that it is safe to use any wattage MagSafe power adapter with any MagSafe-equipped laptop.
This 65-watt MagSafe Power Adapter can be used on a 45-watt MacBook Air without issue.
As discussed on Mac Geek Gab 355, while it may be “safe” to do so, it’s worth noting that it may not always be effective. A higher wattage adapter will charge a lower-wattage computer just fine (for example, an 85-watt 15-inch MacBook Pro adapter used on a 45-watt MacBook Air) but a lower wattage adapter may not charge a higher wattage laptop (a 45-watt MacBook Air adapter connected to an 85-watt 15-inch MacBook Pro). In the former situation, the computer will charge, but will not charge any faster than if the stock adapter is used. In the latter situation, the adapter will not charge the machine at all while in use, and will only trickle-charge the machine while asleep.
All of this information and more is available in article HT2346 over at Apple’s Support Page.