Sunuroo

I'm somewhata novice to the fire emblem series and looking to take on harder modes. As of now, I'm playing causal/normal on birthright. and I'm little confused on the level system. I read people telling me to wait until I reach 20 to level up or People telling me its useless and I'd me missing out on a lot of skills if I don't upgrade at 10. Now this was from a few past fire emblem games like awakening and such. I notice there's a new heart seal and a master seal.