I've pushed most of this from my mind, but with ageing mental illness prevalence does rise considerably.
Specific examples/Main reasons for an increasein mental illness >
- depression: a lack of mobility/independence, death of partner/friends, loneliness, a sense of disconnection from their world around them (e.g. not being able to use the internet, mobile phones, etc), feeling like they've "signed their life away" when entering a nursing home, other mental health problems such as dementia, etc.
- anxiety: a lack of safety/security, target for unsocial behaviour, disability, hormonal changes associated with menopause, etc
NOTE: It is important to talk about both sides of the argument, no matter how small
Decreases in mental illness in the childhood/adolescent/young adult population could come about from factors such as bonds shared with the elderly who have lots of time post retirement, the elderly as free childcare for peace of mind for new parents, etc.
Please note that this is not the answer to your questions, but if you can recognise these reasons out of experience and logic, you can use them as good examples and arguments to back yourself up. PDHPE is not about a concrete answer, but a holistic collection of correspondence and opposing information. Good luck!