Thank you for your question. If the 19 year is moving out this year, you may want to make the adjustment to the W-4 now. This is also assuming that they are not a full-time student in college and they will make over $5,050 in tax year 2024. In that case you, in Step 3 for tax year 2024 would enter $4,000 in one of the W-4s only.
In 2025, if the 19 year old moves out, and they are not a full time student in college and the income is over the income limit for a dependent in 2025 ($ adjusted for inflation). In that case in 2025 you would enter either $2,500 or $4,000. The $2,500 would be entered if the 15 year turns 16 during 2024. If they turn 16 in 2025 you would enter $4,000 in 2025.
Thanks again for your question @Angelique5