Roy Batty was an artificial genetically-engineered being (as explained in the opening text), and these artificial beings (replicants) had been engineered to have a four year lifespan in order to prevent them from having time to develop too many of their own "emotional responses". He was approaching the end of that life (as were his companions), which is why he had come to Earth to talk to Tyrell. So, he was presumably just slightly under four years old (or he may have been just slightly over it if 'four year life span' wasn't precise). From the transcript [here][1]:

>Deckard: [funny look]. [pause] What's this?
>
>Bryant: Nexus 6. Roy Batty. Incept date 2016. Combat model. Optimum self-sufficiency. Probably the leader. This is Zhora. She's trained for an off-world kick-murder squad. Talk about beauty and the beast, she's both. The fourth skin job is Pris. A basic pleasure model. The standard item for military clubs in the outer colonies. They were designed to copy human beings in every way except their emotions. The designers reckoned that after a few years they might develop their own emotional responses. You know, hate, love, fear, anger, envy. So they built in a fail-safe device.
>
>Deckard: Which is what?
>
>Bryant: **Four year life span.**

Whereas J.F. Sebastian was a human who had a fictional disease called "Methuselah's syndrome" that caused him to age prematurely. They put the actor in subtle wrinkly makeup to indicate this, which the actor playing Roy Batty didn't have, so however they engineered the replicants to have four-year life spans, it presumably wasn't by giving them Methuselah's syndrome. From the transcript:

>Pris: Thanks. -- How old are you?
>
>Sebastian: Twenty-five.
>
>Pris: What's your problem?
>
>Sebastian: Methuselah's syndrome.
>
>Pris: What's that?
>
>Sebastian: My glands. They grow old too fast.
>
>Pris: Is that why you're still on earth?
>
>Sebastian: Yeah, I couldn't pass the medical. Anyway, I kind of like it here.


  [1]: http://www.trussel.com/bladerun.htm