Adult learners can depend on a lot of useful resources when they learn a brand spanking new language. They can analyze language in a abstract way. This ability will let them compare patterns and linguistic forms that are similar or different in their sister tongue and in other language.
By contrast, kids cannot make use of these advantages yet, or at least there's significant differences between various age groups in the extent to which they can do so. For example, a class of six-year-olds will be largely unable to reflect on how their first language works, and will show no interest or inclination to notice language forms in either their first or second language. They will pick up and learn the second or foreign language in the event that they are having fun and in the event that they can work out massages from significant contexts. This means learning holistically without attention to abstract language forms. As they progress to older kids, the first language development will permit them increasingly opportunities for useful comparison between the languages they know. Their growing abilities in their sister tongue, for example, to construct phrases, sentence, or questions, generate and retell tales, or to hold conversation, will all be important direct or indirect sources of support in the process of learning another language.