How to estimate the load bearing of MDF plates on a wood/beam construction?
Residential floors in Europe are designed to cope with (by default) either 2.0kN/m2 or a 2kN point load. [Reference: EN 1991-1-1 2002 Tables 6.1 and 6.2].

You are proposing 600kg (= 6kN) over 2m by 1.4m, giving 2.15kN/m2. Considering a normal bed sits on a floor, I'd say we don't need to design for higher loads than a normal floor.

18mm thick MDF floorboards are allowed to span 450mm; 22mm thick MDF floorboards are allowed to span 610mm. [Reference: southern-timber.co.uk] These spans are limited by deflection criteria rather than capacity criteria.

The deflection limit gives a proportional relationship between depth3 and span2. (I'll spare you the engineering equations unless you really care.) Assuming that 15mm MDF is still deflection-governed, and strength doesn't start to dominate, this gives a limiting span of 340mm.