My demihumans are all physically grown around age 20 but are considered adults later than humans. They can be compared to different human age archetypes.
Elves are eternal late teens. From the angst to the enthusiasm, the slim build, the flowering talent, the physical attractiveness and mercurial (chaotic) natures of teens.
Dwarves are eternal 50-year-olds. With a parental attitude towards the other races, a gruff "I know better" attitude, and consummate skill in whatever they specialize in.
Gnomes are eternal 10-year-olds on the inside. They have the endless energy and curiosity of precocious children.
And halflings... are just variant gnomes in my world.
Orcs don't really fit this pattern, as they are the archetype of the hostile "other", that which is seen from the outside and appears alien. No Orc PCs in my games.