Looking at some stats, it seems like the number of suicides does not decrease as age increases through a human's life cycle, if anything it increases slightly especially for men, hitting peak for men over 75. Suicide for women is significantly lower than for men in all age groups, and does slightly decrease as people age, even at 75. But obviously suicide is a much higher cause of death for younger people as younger people are less likely to die from natural causes.
It does seem like the demographic on sasu is very skewed towards very young adults, possibly because younger people are more likely to be on the internet and using a forum or online chat, I imagine the rate of using an internet forum or chat room significantly drops off beyond the age of 40, because people of that age and older didn't grow up with the internet, even if they are just as lonely.
I think the reasons through the ages can be very complex due to different challenges faced by each age group, I don't know if my personal guesses are that relevant because everyone is different.
I'm 30+ personally, I was not suicidal until very recently, due to marriage breakup and abuse by my spouse resulting in cptsd.
Do I think older people are likely to go through with it? Again I think this is a very complex question and I don't know if my guesses mean anything. Personally for me I do feel that as I am older I am more confident is knowing what life has on offer for me in the future, so not as much hope for the unknown. An older person could possibly also carry more regret purely due to longer time spent living, and perhaps more fatigued by life if they have been suicidal for a long time since youth. Divorce especially for men is a known big cause for suicide. You may be more likely to go through with it perhaps also if you are faced with terminal illness as you get older. But again, there r so many factors at play for everyone individually.