If a non-native English speaking 5 year old said "I goed to school today" people would say they are native level. (Even though they used the non-word "goed" as the past tense of "to go" instead of "went")
If a non-native English speaking 15 year old said "I goed to school today" people wouldn't say, "wow, they are a native speaker" nor would they say "wow, they are natively speaking like a 5-year-old" - they would say they still needed practice.
That's my point - speaking like a 5 year old as an adult wouldn't be considered native by anyone but a 5 year old. As you get older the standard is higher: speaking like a 10 year old is much harder than speaking like a 5 year old.
So for a 10 year old just learning a new language it may take longer than the 5 year old to get to their age level. That doesn't necessarily mean the 10 year old is worse at learning languages though since they have so much more to learn to be considered "native".
If a non-native English speaking 15 year old said "I goed to school today" people wouldn't say, "wow, they are a native speaker" nor would they say "wow, they are natively speaking like a 5-year-old" - they would say they still needed practice.
That's my point - speaking like a 5 year old as an adult wouldn't be considered native by anyone but a 5 year old. As you get older the standard is higher: speaking like a 10 year old is much harder than speaking like a 5 year old.
So for a 10 year old just learning a new language it may take longer than the 5 year old to get to their age level. That doesn't necessarily mean the 10 year old is worse at learning languages though since they have so much more to learn to be considered "native".