The age of tectonic plates
The process of forming continental crust is indeed thought to have started in the early Archean (i. e. between ca. 4 Ga and ca. 3.5 Ga). But it did continue later: Hawkesworth &amp; Kemp showed in their 2006 review that while much of the continental crust had formed by 2 Ga, i. e. until the middle of the Paleoproterozoic, some of it still continued forming later.

As far as the Fennoscandian shield is concerned, it seems that though it started forming in the Meso-Archean, and that much of the eastern and northern part of it are Archean (see Slabunov et al. 2006), the southern and western part is indeed mostly Paleo- and Mesoproterozoic (see e. g. Lahtinen et al. 2008). In Figure 1 of Lahtinen et al. 2008, it seems that the southern part of Finland, just north of the Gulf of Finland (the Uusimaa Belt) is Paleoproterozoic. 

Sources:
Hawkesworth &amp; Kemp, 2006. Evolution of the continental crust. Nature, 443: 811-817.
Lahtinen et al., 2008. Paleoproterozoic evolution of Fennoscandia and
Greenland.Episodes, 31: 20-28.
Slabunov et al., 2006. The Archaean nucleus of the Fennoscandian (Baltic) Shield. In Gee &amp; Stephenson (eds): European Lithosphere Dynamics, Geological Society of London, Memoirs, 32: 627-644.