"I am billions of years old yet I look like i'm twenty hehe."

This is a character trait that I find A LOT, and each time it makes me take a second to grumble and rub my eyes as I try not to pull them out. 

Life span is another character trait that come with making OCs, and the longest lifespan I believe I have is for the Starwarriors, that they can live longer because their special soul allows them to. But even then they don't live like, billions of years. The oldest character I know I have (not including 'god' characters who have been around since before time)  would be the creature from  a poem I wrote in ninth grade, depicting a creature called the Beross Wyvren. It's a million years old and frozen in time, hence why it is the age it is. 

Having a very long lifespan should fit into the character's story, and keep in mind you have to fill that age up with events. The longer the lifespan, the more things you have to come up with that the characters done with their life. Having a character be billions of years old brings up the question of 'so what have you done in this time..?' When the character is childish, it asks the question of 'well you've had plenty of time to mature why aren't you mature?' And finally, a planet or a galaxy takes around handful of billion years to form, so what you're practically saying is you've been around since things started. Often these characters have parental figures or parents that have been around even longer, and they are the third or forth generation, saying that these elders would of had to been around since beginning itself. Yet I find that these characters aren't infinite pools of knowledge because nobodies considered that making a character that old means not just considering the character, but the world around them and the universe around them. 

To sum it up, don't make a character 'billions of years old' and have them look like their twenty, because that takes away the meaning of giving a character an age to begin with. When giving a character an age, you need to first consider the rest of the world and what stage it is in. If they look like their twenty, chances are unless you can give a valid reason to why they aren't (for example Percival is 77,000 some years old but to him he's basically still in his 40s is because he is a starwarrior, and his soul ages his body much slower.) you should make them twenty. Having a character billions of years old is just kind of pointless and more often then not adds nothing but a fair bit of stu-sue to them.