You get a slight ability score increase in the areas that your parent races would normally get an increase in. However, due to you being of mixed race, your ability score does not increase as much as a pure-blooded creature would (+1 strength for being Dragonborn instead of +2, +1 Dexterity for being Elvish instead of +2). 

Young dragonborn grow quickly. They walk hours after hatching, attain the size and development of a 10-year-old human child by the age of 3, and reach adulthood by 15. They live to be around 80.

Although elves reach physical maturity at about the same age as humans, the elven understanding of adulthood goes beyond physical growth to encompass worldly experience. An elf typically claims adulthood and an adult name around the age of 100 and can live to be 750 years old.

As a Dragonborn Half-Elf you mature more slowly than a typical dragonborn but much quicker than an elf. You begin walking around 6 months of age. You reach the size of adulthood around age 18, but you don't reach full maturity until age 50. You lifespan is approximately 500 years. 

Dragonborn tend to extremes, making a conscious choice for one side or the other in the cosmic war between good and evil (represented by Bahamut and Tiamat, respectively). Most dragonborn are good, but those who side with Tiamat can be terrible villains.

Elves love freedom, variety, and self-expression, so they lean strongly toward the gentler aspects of chaos. They value and protect others’ freedom as well as their own, and they are more often good than not. The drow are an exception; their exile into the Underdark has made them vicious and dangerous. Drow are more often evil than not.

A Dragonborn Half-Elf may be any alignment but tend to be either Chaotic Good or Chaoitic Evil. 

Dragonborn Half-Elves are taller and heavier than humans, though smaller than a pure-blooded dragonborn. They stand at approximately 6 feet tall and weigh around 200 pounds.

Your base walking speed is 30

You have draconic ancestry. Choose one type of dragon from the Draconic Ancestry table. Your breath weapon and damage resistance are determined by the dragon type, as shown in the table.

You speak Draconic, Elvish, and Common

Accustomed to twilit forests and the night sky, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

You have advantage on saving throws against being charmed, and magic can’t put you to sleep.