  Some theoretical models propose that O-B stars form via accretion, in a
similar fashion to low-mass stars. Jet-driven molecular outflows play an
important role in this scenario, and their study can help to understand the
process of high-mass star formation and the different evolutionary phases
involved. Observations towards low-mass protostars so far favour an
evolutionary picture in which jets are always associated with Class 0 objects
while more evolved Class I/II objects show less evidence of powerful jets. The
present study aims at checking whether an analogous picture can be found in the
high-mass case. The IRAM 30-m telescope (Spain) has been used to perform
single-pointing SiO(2-1) and (3-2) observations towards a sample of 57
high-mass molecular clumps in different evolutionary stages. Continuum data at
different wavelengths, from mid-IR to 1.2 mm, have been gathered to build the
spectral energy distributions of all the clumps and estimate their bolometric
luminosities. SiO emission at high velocities, characteristic of molecular
jets, is detected in 88% of our sources, a very high detection rate indicating
that there is ongoing star formation activity in most of the sources of our
sample. The SiO(2-1) luminosity drops with L/M, which suggests that jet
activity declines as time evolves. This represents the first clear evidence of
a decrease of SiO outflow luminosity with time in a homogeneous sample of
high-mass molecular clumps in different evolutionary stages. The SiO(3-2) to
SiO(2-1) integrated intensity ratio shows only minor changes with evolutionary
state.
