Answer by taking a quote from the following article:

The Gathering is a Dutch rock band, founded in 1989 by brothers Hans and Rene Rutten and vocalist Bart Smits in Oss, North Brabant. The Gathering earliest releases were categorized as atmospheric doom metal with influences from death metal acts like Celtic Frost and Hellhammer. In 1998 with the release of their 5th studio album, How to measure a planet?, they had a major shift in musical style, with the group acknowledging the growing influence of shoegazing, post-rock, experimental rock and the more ethereal sounds of 4AD bands, such as Dead Can Dance, Cocteau Twins, as well as Pink Floyd and Massive Attack; they expanded their sound, which acquired characteristics of progressive, alternative rock, and trip hop and gained success and recognition beyond the European borders. The group continue to expand upon the experimental nature of their music.

Founded by brothers Hans and Rene Rutten and vocalist Bart Smits, The Gathering formed in the city of Oss, North Brabant, Netherlands in 1989. Soon after they were joined by Hugo Prinsen Geerligs, Jelmer Wiersma and Frank Boeijen to complete their first line-up. The Gathering's earliest recordings were categorized as atmospheric doom metal with influences from death metal acts like Celtic Frost and Hellhammer.  In 1990 they recorded a demo tape entitled An Imaginary Symphony which met some positive reactions from the underground metal scene due to their unusual use of keyboards in metal-oriented music. A second demo, "Moonlight Archer" was recorded in April 1991, was picked up by several music journalists. Both demos suffered from poor production; however they did help establish the band's name on the live circuit opening for bands such as Deadhead, Invocator, Samael, Morbid Angel and Death.  After signing with Foundation 2000, the group released their debut album Always... in 1992. On this album, Bart Smits was accompanied by Marike Groot on vocals, who also joined The Gathering on stage for most of the gigs. the album was a minor success on the European continent, where it went on to shift nearly 20,000 units over the next few years. In 1992 both Bart Smits and Marike Groot decided to leave the group due to musical differences; the other members wanted to change direction towards a lighter, more progressive sound. Smits went on to form his own project, called Wish, to explore a darker, heavier sound.  In 1993 the group recruited two new vocalists, Niels Duffhues and female counterpart Martine van Loon. A second album, Almost a Dance, was recorded and released in 1993 by Foundation 2000. The album was met with much criticism aimed at Duffhues' punk-ish tone being decidedly out of step with the music, and the album was largely written off as a result. The group collectively acknowledged their disappointments with Almost a Dance and decided to start writing new material and start the search for a new vocalist.

how old were they when they started the band?