None of the powerups in the game punish you for taking them early:
Attack powerups are a 10% bonus, your damage is actually a base damage amount multiplied by a bonus percentage, the bonus gets increased by powerups. So at level 1, you'll have 5 base + 10% bonus for 5 total damage; At level 4, you'll have 20 base + 10% bonus for 22 total damage. You can see your current base and bonus damage amounts by mousing over (or tapping on) the damage icon under your player portrait.
Health powerups work similarly: They give you a max health increase equal to your current level, but they also add +1 to the amount of health you get when you level up, so picking up a health powerup at level 1 is exactly equivalent to picking one up at level 5 by the time you get to level 5: Level 1 health pickup gives +1 max health, and an extra +1 per level up, so health bonus at level 5 = +5 max health; Level 5 health pickup gives +5 max health. See how that's the same?
And mana powerups, well, they only ever give you +1 mana anyway, so picking them up later doesn't matter
