I am 70. I take the bus in Buenos Aires almost every day. Sometimes I ride 6 in one day, if I have errands far afield. I take the subway pretty often too, its more crowded and, I would say, more susceptible to pickpockets. But I have been taking both since 2007, and never had any incident of any kind myself.
I have taken buses at every hour, including coming back from concerts at 4 am, or with lots of luggage.
I have never seen drunks or the mentally ill making trouble on buses or subways in BA.
I see that pretty much every time I take public transport in the USA.
Once, there was a guy on the colectivo who was drunk, and began singing a tango song. He was pretty good, though, and the whole bus applauded when he finished.
I see people pay the fare of someone who doesnt have a Sube card probably once a week or so.
and I see people giving up seats for the elderly or children daily.
I have taken buses to many of the so-called "dangerous" suburbs in the Conurbano, over the years, always without incident.
Its possible, certainly, to have your cell phone snatched anywhere in BA, and it happens on buses- but the average bus almost always has more women than men riding, ranging in age from 10 to 90 years, and at any given time, I would say over half the people on any given bus are on their phones.
so, no, its not very dangerous.
And its really really cheap.
You do need a Sube card, though.