It follows fears about possible restrictions because of the cost of the Scotland-wide scheme for the over-60s, which is being reviewed by the Scottish Government.
The eligible age south of the Border has increased incrementally from 60 since 2010, based on the increasing female pension age, and will reach 65 in 2020.
The Scottish scheme will cost nearly Â£202 million this year compared to Â£159m when it was launched in 2006, and is expected to increase further as people live longer.
More than 1 million people benefit from the scheme, which represents one third of all bus journeys.
Passenger watchdog body Transport Focus told MSPs that nearly half of those it had polled who were in the scheme had no other way of getting about.
Senior stakeholder manager Robert Samson said: "49 per cent of concessionary travel holders we surveyed say they travel by bus because they do not have any other option.
"It's still a lifeline service in many ways."
Sheila Fletcher, of the Mobility and Access Committee for Scotland, also told Holyrood's rural affairs and connectivity committee: "I would really struggle with any changes because I think what we have at the moment is an excellent system that enables people to get out and about, and it is combating loneliness and isolation.
"It is really important that we try to keep it as it is."
Gavin Booth, director of Bus Users Scotland, told the committee: "As a beneficiary of the scheme, I would hate to see it change in any way.
"I would be sorry to see the lower age range rising as it has done in England, but I understand the sums involved in all of this.
"It's a tremendously useful thing - it gives us mobility, it helps our health.
"I applaud the Scottish Government for introducing it and maintaining it at the level it is.
"I would hate to see anything change."
Transport minister Humza Yousaf has pledged that those in the scheme would not lose their bus passes.
But he said in March there was "concern around the longer-term sustainability" of the scheme.
He said: "We know that we have an ageing population, an ageing demographic - as most of western Europe does - and therefore we have to find a balance between making this scheme fair, realising the benefits of it, and making it sustainable in the long term."