Yep, depends on the plant - all will need hardening off before placing permanently outside, but some of the more tender ones you need to wait till risk of frost is past, which is usually considered to be end of May.

I agree with above comment. But also recommend using a reflector made from kitchen foil to prevent seedling becoming leggy and leaning towards light if they are indooors.

That's ingenious! Or just turn them round every day. Depending on what the plants are you might need to put them in a bigger pot before they are big enough to go out - keep your eye on the drainage hole to check if roots are showing through.

Good advice. Thanks.