Ideally no older than 45, because you have to think of the age at which they will still be financially supporting the DC. If they are very wealthy, then some of that worry goes away and is less relevant, but most people aren't.
If DC goes to uni then you'll be potentially supporting them to age 21-22 and possibly beyond, since many young people struggle to support themselves initially. Someone of 45 will be 70 when that DC is 25. It's important to do the maths.