It is common for bilingual children to have a slower rate of language development in their first language compared to monolingual children. This is because they are learning two languages at the same time, and this can put a strain on their cognitive resources. However, research has shown that bilingual children typically catch up to their monolingual peers by around age five or six in terms of language skills.
It is important to keep in mind that every child is different, and some bilingual children may catch up more quickly than others. Children who are exposed to both languages from a very young age and who have consistent and frequent input in both languages are more likely to catch up more quickly.
It is also important to note that being bilingual does not necessarily mean that a child will be delayed in language development. Many bilingual children develop language skills at a similar rate to monolingual children and may even have some advantages in terms of cognitive flexibility and problem-solving skills.