While any birthday can be fun in its own right, some have special meaning. These will vary depending on your culture and heritage--some numbers have more significance than others for different people. A person's very first birthday is important in many cultures, and although the child won't remember whatever is done to mark the occasion, most people still enjoy doing something to celebrate. Other ages have special meaning depending on where you live. For example, turning 15 is a big deal in Mexico and other Latin American countries where a girl celebrates with a quinceañera, but isn't as important an age in my culture. If there's a particularly extravagant gift you want to give someone, a milestone birthday is a good excuse to give it.
Reaching the next decade of life often stands out as extra reason to celebrate. I have dozens of aunts, uncles, and cousins, which makes celebrating each of their birthdays individually a bit unwieldy, but I make sure to send them cards for their fortieth, fiftieth, sixtieth, seventieth, eightieth, and ninetieth birthdays (we've had a few of the extended family make it to the century mark, but that was before I learned to read and write). If the importance is the number itself, a gift of that number of items can be nice. When my parents turned 60, my siblings and I gave them each a $60 gift card to mark the occasion, more than we usually spend on their birthdays. While you might normally call or visit around a birthday, big milestones can be the impetuous for more people to be able to visit, or for you to save up for a trip to see the person. When my paternal grandfather turned 80, we all gathered for a party for him--and did the same ten years later when he turned 90. A few years after that, there was a similar party on the other side of the family when my maternal grandfather also became a nonagenarian.
Sometimes the reason for extra celebration is new privileges the new age brings. In the US, a person can get a driver's license upon turning 16, becomes a legal adult at 18, and is allowed to buy alcohol at 21. It's pretty common to see gifts relating to cars, independence, and drinking at those ages (of course, be mindful of individual's preferences and possible restrictions; not everyone will be able to or want to partake of the new privileges). It can also be fun to celebrate less well-known perks, like turning 25 and being able to rent a car without purchasing extra insurance, or turning 35 and (in the US) being eligible to run for president.
There are a variety of songs about age. The Silver Beetles song "When I'm 64" asks "Will you still need me, will you still feed me, when I'm 64?" My parents like the song, so I brought them dinner when they hit that age. A currently-teenaged family friend likes Taylor Swift--I'm already looking forward her twenty-second birthday so I can make her a card themed around her song "22." Adele has an album titled 21, Neil Sedaka sang "Happy Birthday Sweet 16." There are also movies related to particular ages or numbers in general that can serve as a springboard for ideas. You might have inside jokes about certain ages that could inspire ideas too.
Golden birthdays are fun to celebrate too. A golden birthday is when a person turns the same age as the date; for example, turning five on the fifth on the month. Of course, people born on the first, second, third, and probably fourth won't remember their golden birthdays, but it can still be a fun excuse for a special gift. I give my godchildren, nieces, and nephews birthstone jewelry on their golden birthdays. The earliest golden birthday so far was when a nephew turned eight; he got a tie bar with his birthstone embedded in it. A goddaughter got a birthstone necklace when she turned twelve, and a niece got birthstone earrings when she turned seventeen. One niece will have to wait until she's 29 for hers!
If you happen to share a birthday with someone you know, that can also be special. One of my daughters was born on my cousin's birthday, and she sends her "birthday twin" a card every year. I also know a few parents whose children were born on their birthdays and other people (aside from twins and other multiples) who share birthdays and enjoy celebrating with the other person.
