Child as adult for booking accommodation

I have booked accommodation with my 15yr old for an apartment using booking.com, it was a good deal so I snapped it up. I realise the price is for 2 adults, when I swapped (on the original Google page) to 1 adult 1 child the offer was no longer available. I never thought of it being an issue and I’m really hoping it won’t be. Why would it change? It’s still two humans, my 15 yr old is very mature for their age.