Prosecutors at the Victorian County Court have called for jail terms for five men who set fire to a mentally ill man last year.

The five Mornington Peninsula men threw petrol on Rosebud man Irving Plotkin and set fire to him.

The court heard Mr Plotkin's injuries were shocking, with more than half of his body badly burnt, and he would be disfigured and severely scarred for the rest of his life.

He was critically injured and more than half of his body was burnt.

The court heard his house was destroyed.

The prosecution told the court the offences were so serious that an immediate jail term in an adult's prison was warranted.

The court heard Mr Plotkin was extremely frightened by the unprovoked attack.

The five men in their late teens and early twenties pleaded guilty to charges including arson and reckless conduct endangering life.

One of the defendants lawyers told the court the attack was a combination of naiveity, boredom, immaturity and stupidity, and there was no desire to cause physical harm.

The court heard one of the men involved in the attack, 20-year-old Adam Taylor of Rosebud, was mortified by his own conduct.

Taylor's lawyer told the court, his client said it was the stupidest thing he had ever done.

The court was told he had at times been friendly with his victim, who could not understand why he had been attacked.

The plea hearing continues.