why can't people over 45 adopt?

What makes you so sure this is the case? I would imagine, however, that older people are not favored as adoptive parents of babies or young kids because they don't have the energy or longevity of younger parents. However, it might be that for older children, such as teens, an older parent, even a single parent might be preferred.

They're maybe some agencies out there that worry that children will be used as hired help when older people get older. Children need a happy, healthy home environment and a good education. They don't want children to become nursemaids for elderly parents at such an early age.

Mainly concerns that the infant may be orphaned a second time due to the parents' advanced ages. Many countries have done away with strict age limitations, though prospective parents' overall health does play a role in whether they are approved to adopt. 

It could be that aging and death stuff.

I suppose they think that old people don't have the physical health to look after children for the next 18 years.  The thing is that many young people are really unfit these days.  It should be based on general fitness not age.  I'm 57 and I'm much fitter and "younger" than many people half my age.  People who use age cut-offs like that may have good intentions but are really just being lazy.